Chopped the barrel on my Ruger Explorer
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jcwit
member
How long is "a lot"?
I've been shooting Beeman's & H&W's since the 1980's.
Ever own or shoot a Feinwerkbau 300S?
The scope doesn't care about the double recoil, it cares about the G's which are only in the second, aka forward, aka reverse recoil which is caused by piston hitting the breech or end of the tube depending on what you call it. The recoil is soft and smooth like a firearm because it takes place over time, the reverse one is abrupt which is why it's high G. Swinging your fist to full speed is painless, an abrupt stop on steel is not. G's...
I remember Beeman bragging about how their scopes could handle a full auto, which they demonstrated, but what an incredible joke to play on their naive customers. From a business standpoint it was great since it sold scopes, but useless without public ignorance. I suppose rather than explain all the details about how and why, it was just easier for the mfg to say double recoil breaks scopes. Like the artillery hold is easier than explaining the how and why, or silicone chamber oil is easier than explaining the list of oils that are and are not ok.
And yes the R1 piston slams into the breech, which is why it broke scopes, but I'm curious what makes you think it isn't? What forces other than that could possibly explain a scope sliding aft on the rail? I've seen scope stop pins cut a trench in the rail, which takes tremendous force. So if not piston impact where does does it come from?
Many variables play a part in how hard it hits, like spring strength, bore/stroke piston weight, transfer port size, dead air space in the seal, and pellet weight. Some guns no doubt don't impact, but I don't own any. Well, I do have one that might not but it's a special case.
And yes that is what the dry firing thing is all about. Without a pellet to allow pressure to build up and slow the impact, it will hit with even more force which is obviously a bad thing. Light pellets are not as good as heavies in that regard, but light alloy will also have a much stronger skirt so that will take more pressure to force into the barrel. How much it helps I can't say.
I don't follow on that spring collapsing thing mentioned earlier, but seems you understood it.
Thanks Crawfish
I remember Beeman bragging about how their scopes could handle a full auto, which they demonstrated, but what an incredible joke to play on their naive customers. From a business standpoint it was great since it sold scopes, but useless without public ignorance. I suppose rather than explain all the details about how and why, it was just easier for the mfg to say double recoil breaks scopes. Like the artillery hold is easier than explaining the how and why, or silicone chamber oil is easier than explaining the list of oils that are and are not ok.
And yes the R1 piston slams into the breech, which is why it broke scopes, but I'm curious what makes you think it isn't? What forces other than that could possibly explain a scope sliding aft on the rail? I've seen scope stop pins cut a trench in the rail, which takes tremendous force. So if not piston impact where does does it come from?
Many variables play a part in how hard it hits, like spring strength, bore/stroke piston weight, transfer port size, dead air space in the seal, and pellet weight. Some guns no doubt don't impact, but I don't own any. Well, I do have one that might not but it's a special case.
And yes that is what the dry firing thing is all about. Without a pellet to allow pressure to build up and slow the impact, it will hit with even more force which is obviously a bad thing. Light pellets are not as good as heavies in that regard, but light alloy will also have a much stronger skirt so that will take more pressure to force into the barrel. How much it helps I can't say.
I don't follow on that spring collapsing thing mentioned earlier, but seems you understood it.
Thanks Crawfish
There is a matter of degrees, here. I agree that if the piston hits the breech at high speed, let alone at its peak velocity, the G's could be much higher than the "air cushion model."
I believe that many, most, or even in all air rifles, the piston may be slamming into the breech at close to maximum velocity. Sure, why not.
But OTOH, I still wonder if you aren't underestimating the G's that could be created by air.
To put things into perspective, a spring rifle creates (much?) more pressure than a PCP at 3000 psi. Magnum springers can visibly blow out and deform the skirt of pellets in a way that PCP's do not. And that pressure spikes very rapidly, as volume decreases dramatically and temperature rises dramatically. The piston is rapidly accelerated by the spring and that train is still accelerating when this mess is already starting to go the wrong way.
I know what you're saying about the powder burner having even 50k+ psi. But that 50k+ psi rifle needs about 2" of barrel to accelerate through the meaty part of its curve and to reach that pressure, so it's not, as you say, as high G as the air rifle. But the bullet weighs only on the order of 100-200 grains, lets say, for an average centerfire, and the bullet and rifle are both smoothly accelerating away from that area of pressure, not towards it. The spring and piston of the air rifle probably weighs many times than a bullet, moving into what turns into several thousands of psi in a very short time frame. And mind you I'm just guessing here, but my active imagination thinks that the meat and potatoes of the deceleration phase of the piston can occur in a much shorter span than 2", even when by air, alone. I can picture 90% of the deceleration occurring in a small fraction of an inch. But that's just my imagination. I could be quite wrong.
I believe the seal has to cover the front of the piston, or else it would slip off. But I understand what you're saying about the seal being flattened like a pancake, as if by hammering. If the seal were stopped by air, it might be compressed, but it would not take the shape of the breech... although it might, due to standing uncocked long enough. Hmmm, a matter of degrees, again.
All this has me wondering what kind of damage you could do with an air rifle by cutting off the breech and firing the piston and spring, lol.
I also see you have a good point about the double recoil. In fact, most centerfire rifles do not need scope stops. So even disregarding the fact that the recoil occurs in the opposite direction, it would seem to follow that springers still put more G's on a scope than a powder burner.
Anyhow, if I ever take apart my rifle, I will PM you the specs on the spring. I am hitting fingernail sized stones 95% of the time at 9 yards, now. I'm very happy about that, and the tree rats would be in big trouble if my oranges were in season. But I still see some room for improvement.
I believe that many, most, or even in all air rifles, the piston may be slamming into the breech at close to maximum velocity. Sure, why not.
But OTOH, I still wonder if you aren't underestimating the G's that could be created by air.
To put things into perspective, a spring rifle creates (much?) more pressure than a PCP at 3000 psi. Magnum springers can visibly blow out and deform the skirt of pellets in a way that PCP's do not. And that pressure spikes very rapidly, as volume decreases dramatically and temperature rises dramatically. The piston is rapidly accelerated by the spring and that train is still accelerating when this mess is already starting to go the wrong way.
I know what you're saying about the powder burner having even 50k+ psi. But that 50k+ psi rifle needs about 2" of barrel to accelerate through the meaty part of its curve and to reach that pressure, so it's not, as you say, as high G as the air rifle. But the bullet weighs only on the order of 100-200 grains, lets say, for an average centerfire, and the bullet and rifle are both smoothly accelerating away from that area of pressure, not towards it. The spring and piston of the air rifle probably weighs many times than a bullet, moving into what turns into several thousands of psi in a very short time frame. And mind you I'm just guessing here, but my active imagination thinks that the meat and potatoes of the deceleration phase of the piston can occur in a much shorter span than 2", even when by air, alone. I can picture 90% of the deceleration occurring in a small fraction of an inch. But that's just my imagination. I could be quite wrong.
I believe the seal has to cover the front of the piston, or else it would slip off. But I understand what you're saying about the seal being flattened like a pancake, as if by hammering. If the seal were stopped by air, it might be compressed, but it would not take the shape of the breech... although it might, due to standing uncocked long enough. Hmmm, a matter of degrees, again.
All this has me wondering what kind of damage you could do with an air rifle by cutting off the breech and firing the piston and spring, lol.
I also see you have a good point about the double recoil. In fact, most centerfire rifles do not need scope stops. So even disregarding the fact that the recoil occurs in the opposite direction, it would seem to follow that springers still put more G's on a scope than a powder burner.
Anyhow, if I ever take apart my rifle, I will PM you the specs on the spring. I am hitting fingernail sized stones 95% of the time at 9 yards, now. I'm very happy about that, and the tree rats would be in big trouble if my oranges were in season. But I still see some room for improvement.
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jcwit
member
Chevota, believe what you wish, one of the joys of living in the US.
The numbers I've seen suggest that the pressure in the chamber peaks around 1000 to 1500 psi.
Why the effect on the skirts? I suspect that one or both of the following is true:
The pressure peak created by the compression is more abrupt than the one caused by a PCP valve releasing.
AND/OR
The pressure level in the area behind the pellet is higher in a springer even though the pressure in a PCP tank is higher than the pressure generated by a springer. In other words, the fact that there's 3000psi in the PCP tank doesn't mean that the pressure behind the pellet actually ever gets that high. The valve release isn't instantaneous and there are pressure losses in rapidly moving the air from the tank to the area behind the pellet.
I don't have any data relating directly to the R1, but the Cardews did some testing which indicated that in a properly operating springer, the piston typically stops just short of hitting the end of the breech and rebounds a short distance. The piston travel curve they obtained indicated that the piston, which had been moving at about 40fps reversed travel in roughly half a millisecond. The fact that the piston hit an "air cushion" instead of the front of the chamber doesn't mean there won't be a lot of acceleration generated.
It is not difficult to calculate how much acceleration is required to stop the piston in half a millisecond. The number I obtained from my back of the envelope scribbling, assuming a piston weight of 3/4 of a pound, was quite impressive.
jcwit
member
Well, Glory Be!
"How long is "a lot"? 20,000 + rounds in the last year or 40 tins
I've been shooting Beeman's & H&W's since the 1980's. I lived near the Beeman warehouse in Santa Rosa Ca cool place and worked in San Rafael where they were distributed from. Really nice area but super expensive.
Ever own or shoot a Feinwerkbau 300S?" No never have. Have you shot a FX Bobcat or a Edgun Matador I really like mine.
I've been shooting Beeman's & H&W's since the 1980's. I lived near the Beeman warehouse in Santa Rosa Ca cool place and worked in San Rafael where they were distributed from. Really nice area but super expensive.
Ever own or shoot a Feinwerkbau 300S?" No never have. Have you shot a FX Bobcat or a Edgun Matador I really like mine.
"I don't have any data relating directly to the R1, but the Cardews did some testing which indicated that in a properly operating springer, the piston typically stops just short of hitting the end of the breech and rebounds a short distance. The piston travel curve they obtained indicated that the piston, which had been moving at about 40fps reversed travel in roughly half a millisecond. The fact that the piston hit an "air cushion" instead of the front of the chamber doesn't mean there won't be a lot of acceleration generated".
"Properly Operating"
I know quality control on HW's has gone down in recent years. The manufacturer has left burrs on the compression cylinder, pieces of metal in the gun and so on. So maybe this is the reason some are harder on scopes than others.
"It is not difficult to calculate how much acceleration is required to stop the piston in half a millisecond. The number I obtained from my back of the envelope scribbling, assuming a piston weight of 3/4 of a pound, was quite impressive."
___ I am confused you must have meant deceleration?
If you suggest a piston is going to stop on a cushion of air and travel backwards in a millisecond I would have to assume that in a properly working air gun, timing of said compression stroke and design of the barrel length would be paramount in keeping the piston from bottoming out in it's bore.
Using this theory it fully supports Chevota's theory of Gloob's piston slamming into the bore as he cut a great length of of his factory barrel which would effect the compression timing of his rifle.
Either way it's a interesting discussion and until someone takes a gun apart and puts a transfer media to mark if the piston touches it's bore I will keep my mind open. Seeing how this may vary manufacturer to manufacturer , model to model, design to design it's going to be really subjective to individual circumstance and quality control.
"Properly Operating"
I know quality control on HW's has gone down in recent years. The manufacturer has left burrs on the compression cylinder, pieces of metal in the gun and so on. So maybe this is the reason some are harder on scopes than others.
"It is not difficult to calculate how much acceleration is required to stop the piston in half a millisecond. The number I obtained from my back of the envelope scribbling, assuming a piston weight of 3/4 of a pound, was quite impressive."
___ I am confused you must have meant deceleration?
If you suggest a piston is going to stop on a cushion of air and travel backwards in a millisecond I would have to assume that in a properly working air gun, timing of said compression stroke and design of the barrel length would be paramount in keeping the piston from bottoming out in it's bore.
Using this theory it fully supports Chevota's theory of Gloob's piston slamming into the bore as he cut a great length of of his factory barrel which would effect the compression timing of his rifle.
Either way it's a interesting discussion and until someone takes a gun apart and puts a transfer media to mark if the piston touches it's bore I will keep my mind open. Seeing how this may vary manufacturer to manufacturer , model to model, design to design it's going to be really subjective to individual circumstance and quality control.
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jcwit
member
Lets all go out and buy a Gamo.
It's always possible to make something wrong/shoddily and then it might not work properly. The point isn't that all airguns work as they should, it's that it's unwarranted to assume that the piston always slams the front of the chamber, nor is that assumption necessary to explain the effects of dual recoil.
http://www.physicstutorials.org/home/mechanics/1d-kinematics/acceleration
"In physics we use acceleration concept a little bit different from its daily life usage. If there is a change in the velocity whether it is slowing down or speeding up, or changing its direction we say that object is accelerating."
The compression peak occurs at, or immediately before, the point when the pellet starts moving which means that the only significant effects on it are the weight of the pellet and the amount of friction initially holding it in place. Changing the barrel length won't have any significant effect.
The piston travel plot shows the piston reversing direction in well under a millisecond. I'm saying half a millisecond to avoid getting into a hair-splitting argument but I think the real value is smaller still.
That wouldn't work. Remember that there's a big, strong spring behind the piston. That means that with the gun in the uncocked state, the piston is resting against the front of the chamber. The piston always comes to rest against the front of the chamber eventually after bouncing backwards initially, but the bounce back distance isn't very large at all and therefore the speed of the piston when it finally does hit the front of the chamber isn't significant.
In order to understand what's happening, it is necessary to rig up a test setup that tracks the piston movement during firing as the Cardews did.
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Jeeze, what a mess this has turned into.
I think people missed the part where I said the seal compresses a good .007" or so, which would take quite the impact to make happen.
The Cardew book is based on guns making maybe 5ftlbs power so as long as the seal and port were good I'm sure the gun did stop on air. I have one that makes maybe 9ftlbs and I suspect it may stop on air too, maybe. If it is hitting it isn't very hard. Push that past 12 and up to 30ftlbs like most all guns sold today and I think they all hit. Plus the way many seals are made they don't allow peak pressure to build, which means harder impact.
Stopping on a cushion of air cannot do what is happening to scopes and mounts. Imagine how hard you'd need to hit a gun with a hammer to get a securely clamped scope with a stop pin to slide and the pin cut a groove in the rail?
I think people missed the part where I said the seal compresses a good .007" or so, which would take quite the impact to make happen.
The Cardew book is based on guns making maybe 5ftlbs power so as long as the seal and port were good I'm sure the gun did stop on air. I have one that makes maybe 9ftlbs and I suspect it may stop on air too, maybe. If it is hitting it isn't very hard. Push that past 12 and up to 30ftlbs like most all guns sold today and I think they all hit. Plus the way many seals are made they don't allow peak pressure to build, which means harder impact.
Stopping on a cushion of air cannot do what is happening to scopes and mounts. Imagine how hard you'd need to hit a gun with a hammer to get a securely clamped scope with a stop pin to slide and the pin cut a groove in the rail?
Regarding my shortened barrel, I did it with the understanding that if the cardew testing can be extrapolated to my gun, the piston is done doing its thing before the pellet acceleration is over. And the latter is done by 6-9" per cardew testing.
I think the piston is doing the same thing it did before. I can't feel any difference. The main difference I can detect is a little bit of muzzle blast noise that is evident when discharging the gun into the backstop at near point blank.
My pellets of choice sometimes flip out of the barrel, so I have experienced a dryfire. It is distinct but not too different. A little louder in muzzle blast oddly.
I think the piston is doing the same thing it did before. I can't feel any difference. The main difference I can detect is a little bit of muzzle blast noise that is evident when discharging the gun into the backstop at near point blank.
My pellets of choice sometimes flip out of the barrel, so I have experienced a dryfire. It is distinct but not too different. A little louder in muzzle blast oddly.
jcwit
member
It can being as most scopes are made to protect against recoil pushing to the rear, its only been recent that scopes have been made to protect the lenses from pushing forward.
Curiously, I think it could go the other way. In my imagination, a lower powered gun might not be able to achieve the amount of rapid compression needed to hit this "brick wall." The pellet might get pushed significantly down the bore before that happened, because the compression isn't as rapid. In my imagination, I think the more powerful guns are the ones that are more likely able to reach this point, given a big enough compression tube. There's no limit to how high the pressure can get, thus no spring that is "too strong" to be stopped by air. The pellet has a discrete amount of inertia, and make the piston faster, you will get more compression. There's a point where even the constriction of the empty bore itself could conceivably produce this "air cushion", given a big enough compression tube and a fast enough piston.
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Some more opinions:
How spring-piston rifles behave
Posted on September 7, 2011 by B.B. Pelletier ↓ 69 Comments
by B.B. Pelletier
Okay, Grasshopper, enough Wax on! Wax off! It’s time to use your skills.
If you’ve been following the discussions over the past month about accuracy, you should now have the tools to be a pretty good judge of the potential accuracy of an air rifle and the relative ease with which that accuracy comes — even before taking the first shot. We’ll confine today’s discussion to just spring-piston guns, since they’re the most difficult to shoot.
How a spring-piston airgun works
This is a review for many of you, but we have enough new readers that perhaps it’s good to go over the points of how the spring-piston gun works. What I’m about to say holds true for guns with gas springs as well as guns with coiled steel mainsprings. They all work the same when it comes to their operation.
When the sear releases the piston, the piston starts moving forward rapidly at 50-60 miles per hour or 73-88 f.p.s. Unless there’s something like an anti-recoil mechanism to prevent it, the gun starts moving in the opposite direction. Since the piston weighs but a fraction of the weight of the whole gun, the gun’s movement is very slight.
Within a few hundredths of an inch of the end of its travel, the piston has compressed the air in front of it as high as it will ever go…given the piston diameter and length of the piston stroke. Due to this compression, the temperature of the air has also increased to a very high point. The piston wants to slam into the end of the compression chamber, but the thin cushion of highly compressed air actually slows it down and can even stop it. The pellet in the breech is sealing the air in front of the piston, and it hasn’t started moving yet.
However, at some point — and that point changes with each pellet used, the pellet can no longer remain stationary. There’s too much force pushing on its tail and it begins to move down the bore. The piston can now go all the way forward and rest against the end of the compression chamber, or it may have done so already and rebounded off the air cushion and now needs to go forward again. Each different type of pellet will determine exactly how this relationship of movement plays out, which is why some pellets feel good when you shoot them and other pellets seem to make the gun buzz and vibrate and even make noises that you may never have heard before.
When the piston reaches the end of its travel, it stops suddenly. When that happens, it imparts a hammer blow to the airgun, sending it in the same direction the piston was traveling. This is the second recoil, and it’s much more noticeable. At this point in time, the pellet is probably between three and six inches down the barrel and the entire gun’s moving.
The movement is in several forms. First, there’s high-speed vibration running through all the parts of the gun. You can’t see this vibration, even on a high-speed camera, but you can feel it. This is the buzz that you feel from some guns, and it can be so sharp that it actually hurts to hold the stock against your cheek.
Next, there’s a lower-speed vibration that’s both larger and much slower. If you had a high-speed camera, you could actually see the various parts of the rifle moving. The pellet is still inside the barrel when this happens.
Finally, there’s the recoil in both directions. Both are visible on a high-speed camera; and the forward movement, assuming we’re talking about a conventional spring-piston setup, is by far the largest. The gun starts moving forward before the pellet leaves the muzzle, but completes the movement after the pellet has gone.
Full article here http://www.pyramydair.com/blog/2011/09/how-spring-piston-rifles-behave/
Here is another:
The different methods of powering an air gun can be broadly divided into 3 groups: spring-piston, pneumatic, and CO2. These methods are used in both air rifles and air pistols.[6]
Spring-piston[edit]
A typical break-barrel, spring-piston air rifle
Spring-piston air guns are able to achieve muzzle velocities near or greater than the speed of sound from a single stroke of a cocking lever or the barrel itself. The effort required for the cocking stroke is usually related to the power of the gun, with higher muzzle velocities requiring greater effort.
Spring-piston guns operate by means of a coiled steel spring-loaded piston contained within a compression chamber, and separate from the barrel. Cocking the gun causes the piston assembly to compress the spring until the rear of the piston engages the sear. The act of pulling the trigger releases the sear and allows the spring to decompress, pushing the piston forward, thereby compressing the air in the chamber directly behind the pellet. Once the air pressure has risen enough to overcome any static friction and/or barrel restriction holding the pellet, the pellet moves forward, propelled by an expanding column of air. All this takes place in a fraction of a second, during which the air undergoes adiabatic heating to several hundred degrees and then cools as the air expands.
Spring-piston guns have a practical upper limit of 1250 ft/s (380 m/s) for .177 cal (4.5 mm) pellets. Higher velocities cause unstable pellet flight and loss of accuracy. This is due to the extreme buffeting caused when the pellet reaches and passes transonic speed, then slows back down and goes through it again. This is more than enough to destabilize it. Shortly after leaving the barrel, the supersonic pellet falls back below the speed of sound and the shock wave overtakes the pellet, causing its flight to be disrupted. Drag increases rapidly as pellets are pushed past the speed of sound, so it is generally better to increase pellet weight to keep velocities subsonic in high-powered guns. Sonic crack from the pellet as it moves with supersonic speed also makes the shot louder sometimes making it possible to be mistaken for firearm discharge. Many shooters have found that velocities in the 800–900 ft/s (240–270 m/s) range offer an ideal balance between power and pellet stability.
Most spring piston guns are single-shot breech-loaders by nature, but multiple-shot guns have become more common in recent years. Spring guns are typically cocked by a mechanism requiring the gun to be hinged at the midpoint (called a break barrel), with the barrel serving as a cocking lever. Other systems that are used include side levers, under-barrel levers, and motorized cocking, powered by a rechargeable battery.
Spring piston guns, especially high-powered ones, recoil as a result of the forward motion of the piston. Although the recoil is less than that of some cartridge firearms, it can make the gun difficult to shoot accurately as the recoil forces are in effect whilst the pellet is still traveling down the barrel. Spring gun recoil has a sharp forward movement too, caused by the piston hitting the forward end of the chamber when the spring has fully expanded. These two reactions are known to damage scopes not rated for spring gun use.
Spring guns can also suffer from spring vibrations that reduce accuracy. These vibrations can be controlled by adding features like close-fitting spring guides or by aftermarket tuning done by "air-gunsmiths" who specialize in air gun modifications. A common modification is the addition of viscous silicone grease to the spring, which both lubricates it and dampens vibration.
The better quality spring air guns can have very long service lives, being simple to maintain and repair. Because they deliver the same energy on each shot, their trajectory is consistent. Most Olympic air gun matches through the 1970s and into the 1980s were shot with spring-piston guns, often of the opposing-piston recoil-eliminating type. Beginning in the 1980s, guns powered by compressed, liquefied carbon dioxide began to dominate competition. Today, the guns used at the highest levels of competition are powered by compressed air.
Full article here: https://en.wikipedia.org/wiki/Air_gun
Sounds like a lot of this conjecture is based on what pellet an individual is using.
Gotta go shoot my Gamo @ my hundred yard targets now. I need to work on my clean trigger pull seeing as it's about 14 LBS.
How spring-piston rifles behave
Posted on September 7, 2011 by B.B. Pelletier ↓ 69 Comments
by B.B. Pelletier
Okay, Grasshopper, enough Wax on! Wax off! It’s time to use your skills.
If you’ve been following the discussions over the past month about accuracy, you should now have the tools to be a pretty good judge of the potential accuracy of an air rifle and the relative ease with which that accuracy comes — even before taking the first shot. We’ll confine today’s discussion to just spring-piston guns, since they’re the most difficult to shoot.
How a spring-piston airgun works
This is a review for many of you, but we have enough new readers that perhaps it’s good to go over the points of how the spring-piston gun works. What I’m about to say holds true for guns with gas springs as well as guns with coiled steel mainsprings. They all work the same when it comes to their operation.
When the sear releases the piston, the piston starts moving forward rapidly at 50-60 miles per hour or 73-88 f.p.s. Unless there’s something like an anti-recoil mechanism to prevent it, the gun starts moving in the opposite direction. Since the piston weighs but a fraction of the weight of the whole gun, the gun’s movement is very slight.
Within a few hundredths of an inch of the end of its travel, the piston has compressed the air in front of it as high as it will ever go…given the piston diameter and length of the piston stroke. Due to this compression, the temperature of the air has also increased to a very high point. The piston wants to slam into the end of the compression chamber, but the thin cushion of highly compressed air actually slows it down and can even stop it. The pellet in the breech is sealing the air in front of the piston, and it hasn’t started moving yet.
However, at some point — and that point changes with each pellet used, the pellet can no longer remain stationary. There’s too much force pushing on its tail and it begins to move down the bore. The piston can now go all the way forward and rest against the end of the compression chamber, or it may have done so already and rebounded off the air cushion and now needs to go forward again. Each different type of pellet will determine exactly how this relationship of movement plays out, which is why some pellets feel good when you shoot them and other pellets seem to make the gun buzz and vibrate and even make noises that you may never have heard before.
When the piston reaches the end of its travel, it stops suddenly. When that happens, it imparts a hammer blow to the airgun, sending it in the same direction the piston was traveling. This is the second recoil, and it’s much more noticeable. At this point in time, the pellet is probably between three and six inches down the barrel and the entire gun’s moving.
The movement is in several forms. First, there’s high-speed vibration running through all the parts of the gun. You can’t see this vibration, even on a high-speed camera, but you can feel it. This is the buzz that you feel from some guns, and it can be so sharp that it actually hurts to hold the stock against your cheek.
Next, there’s a lower-speed vibration that’s both larger and much slower. If you had a high-speed camera, you could actually see the various parts of the rifle moving. The pellet is still inside the barrel when this happens.
Finally, there’s the recoil in both directions. Both are visible on a high-speed camera; and the forward movement, assuming we’re talking about a conventional spring-piston setup, is by far the largest. The gun starts moving forward before the pellet leaves the muzzle, but completes the movement after the pellet has gone.
Full article here http://www.pyramydair.com/blog/2011/09/how-spring-piston-rifles-behave/
Here is another:
The different methods of powering an air gun can be broadly divided into 3 groups: spring-piston, pneumatic, and CO2. These methods are used in both air rifles and air pistols.[6]
Spring-piston[edit]
A typical break-barrel, spring-piston air rifle
Spring-piston air guns are able to achieve muzzle velocities near or greater than the speed of sound from a single stroke of a cocking lever or the barrel itself. The effort required for the cocking stroke is usually related to the power of the gun, with higher muzzle velocities requiring greater effort.
Spring-piston guns operate by means of a coiled steel spring-loaded piston contained within a compression chamber, and separate from the barrel. Cocking the gun causes the piston assembly to compress the spring until the rear of the piston engages the sear. The act of pulling the trigger releases the sear and allows the spring to decompress, pushing the piston forward, thereby compressing the air in the chamber directly behind the pellet. Once the air pressure has risen enough to overcome any static friction and/or barrel restriction holding the pellet, the pellet moves forward, propelled by an expanding column of air. All this takes place in a fraction of a second, during which the air undergoes adiabatic heating to several hundred degrees and then cools as the air expands.
Spring-piston guns have a practical upper limit of 1250 ft/s (380 m/s) for .177 cal (4.5 mm) pellets. Higher velocities cause unstable pellet flight and loss of accuracy. This is due to the extreme buffeting caused when the pellet reaches and passes transonic speed, then slows back down and goes through it again. This is more than enough to destabilize it. Shortly after leaving the barrel, the supersonic pellet falls back below the speed of sound and the shock wave overtakes the pellet, causing its flight to be disrupted. Drag increases rapidly as pellets are pushed past the speed of sound, so it is generally better to increase pellet weight to keep velocities subsonic in high-powered guns. Sonic crack from the pellet as it moves with supersonic speed also makes the shot louder sometimes making it possible to be mistaken for firearm discharge. Many shooters have found that velocities in the 800–900 ft/s (240–270 m/s) range offer an ideal balance between power and pellet stability.
Most spring piston guns are single-shot breech-loaders by nature, but multiple-shot guns have become more common in recent years. Spring guns are typically cocked by a mechanism requiring the gun to be hinged at the midpoint (called a break barrel), with the barrel serving as a cocking lever. Other systems that are used include side levers, under-barrel levers, and motorized cocking, powered by a rechargeable battery.
Spring piston guns, especially high-powered ones, recoil as a result of the forward motion of the piston. Although the recoil is less than that of some cartridge firearms, it can make the gun difficult to shoot accurately as the recoil forces are in effect whilst the pellet is still traveling down the barrel. Spring gun recoil has a sharp forward movement too, caused by the piston hitting the forward end of the chamber when the spring has fully expanded. These two reactions are known to damage scopes not rated for spring gun use.
Spring guns can also suffer from spring vibrations that reduce accuracy. These vibrations can be controlled by adding features like close-fitting spring guides or by aftermarket tuning done by "air-gunsmiths" who specialize in air gun modifications. A common modification is the addition of viscous silicone grease to the spring, which both lubricates it and dampens vibration.
The better quality spring air guns can have very long service lives, being simple to maintain and repair. Because they deliver the same energy on each shot, their trajectory is consistent. Most Olympic air gun matches through the 1970s and into the 1980s were shot with spring-piston guns, often of the opposing-piston recoil-eliminating type. Beginning in the 1980s, guns powered by compressed, liquefied carbon dioxide began to dominate competition. Today, the guns used at the highest levels of competition are powered by compressed air.
Full article here: https://en.wikipedia.org/wiki/Air_gun
Sounds like a lot of this conjecture is based on what pellet an individual is using.
Gotta go shoot my Gamo @ my hundred yard targets now. I need to work on my clean trigger pull seeing as it's about 14 LBS.
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jcwit
member
Love it!
The Cardews experimented with a variety of energy levels but does appear to be true that most of their work was oriented towards exploring airguns that didn't exceed the FAC limitations.
That said, the idea that making a gun more powerful will increase the chances of the piston hitting the front of the cylinder doesn't necessarily follow from the physics.
A more powerful airgun means more pressure behind the pellet. More pressure behind the pellet means more force acting to slow the piston. Remember, the pressure acts on both the piston and the pellet.
1300psi acting on the front of a piston with a diameter of approximately 1 inch generates over 1,000 pounds of force on the front of the piston to slow it. Increase the power of the gun to the point that now there's 1600psi in the chamber and that means that there's now over 1,200 lbs of force slowing the piston as it approaches the end of the chamber. Up the pressure to 2000psi and now there's close to 1,600 lbs of force opposing the spring and the piston momentum at the end of piston travel.
If the piston is generating over 1,000psi that means it is generating forces of more than a thousand pounds acting against the front of the piston and the seal. That's enough to cause significant seal compression even without any impact being involved. Also, when the gun is uncocked, the seal rests against the front of the cylinder under the resting pressure from the spring which can also result in compression over time.
How many Gs are created by stopping a 3/4 lb piston from 40fps in half a millisecond on a "cushion of air"? It's not a difficult calculation and I think you would find the results very interesting.
Calculate the piston momentum by multiplying the piston weight by the piston velocity. Then from the piston momentum calculate the force required to stop it in 0.0005 seconds since force is equal to momentum divided by time. Then calculate the acceleration since acceleration is equal to the force applied to the piston divided by the mass of the piston. Then divide the acceleration value by the value of G to find out how many Gs of acceleration are required. If you do everything in the metric system things work out very simply with no need to convert pounds to slugs, etc.
I think that you'll find that even people who understand the difference often refer to the piston hitting the front of the chamber. In practice, the difference isn't nearly as significant as one might believe. The combination of increasing pressure and temperature and the feedback that creates which in turn generates more pressure and higher temperatures and so on means that the pressure rises VERY rapidly. That's why the piston travel diagram in the Cardew book shows no significant slowing of the piston until it's within a fraction of an inch of the chamber end.
So the piston stops VERY abruptly even if it is stopped by a "cushion of air". I think that referring to a "cushion of air" can tend to confuse the issue. A cushion of 1300+psi air is not very cushiony.
Even if the piston does hit the chamber end at the end of the initial forward stroke, the pressure/force figures show that it will have been slowed dramatically by air pressure before that occurs. In other words, whether it hits the front of the chamber or not, the huge majority of the work of stopping it is done by the air pressure in the chamber.
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I have an interesting observation on a similar phenomenon: trying to pound a squib bullet out of a bore. I braced the butt of the revolver against a wood benchtop. Using heavy blows with a 3 lb hammer, the slug wouldn't budge, but I could make a good indentation in the bench in the shape of a revolver grip. Conversely, rapping the rod with a 12 oz hammer, the slug easily advanced a teeny bit with each tap without damaging the bench or the revolver.
The key here is the scope is moving only a tiny fraction of an inch at a time, maybe a mil or less. This is possible due to the high peak force applied in the exact right direction. The fact that it's a double/reverse recoil aligns the force exactly right to make the "tap" act exactly in line with the scope rails without the rotational torque that would normally be applied. The fast and light "hammer" not only applies enough force to quickly move the rifle a teeny bit, but it is also enough to send a shockwave through the steel, allowing some vibration/deformation/wiggle of the steel. Just enough to allow the scope mount to slip fraction of a mil through what is an essentially an interference fit.
So, with the right (light) hammer, and applied in the right way, and at the right speed (40-50mph+, like the piston), and repeated hundreds of times, like firing a rifle, then it should be pretty "easy" to replicate.
I haven't read the Cardew book. Only read some references to it. But everything John has said jives with what I have imagined in my head from my understanding of physics and my real world observations. It seems plausible, to me.
If compression of the air didn't take up a big part of the energy that was stored in the spring, the spring rifle would be horribly inefficient. Not that it would necessarily need to be efficient to work. But from an engineering perspective, I just would hope there would be a way to make it as efficient as I imagine it might be in my head. My natural bias WANTS the Cardew testing to be correct, but as I said, I really don't know one way or the other.
There's also the possibility that the rifles that are cutting grooves and/or breaking scope stops are either flawed in design or bad examples/tunes. And they are hitting the breech with too much leftover force.
From my perspective as a redneck engineer, when my rifle was shooting as poorly as it did, and after I exhausted the obvious and easy pickings, I started considering major design flaws. In my mind, when a rifle shoots as badly as that, it's not a matter of putting tar on the spring. It's not a matter of applying moly somewhere. It's not the barrel being "bad" because it's not made in Germany. It's not a matter of Chinese quality control/materials/workmanship. When I can shoot spitballs with greater accuracy than a rifle that doesn't have obvious, observable problems, it is probably something fundamentally wrong with the design. And in my mind, the only thing would have been recoil acting on the pellet before it left the barrel. Maybe something is fundamentally out of whack with those rifles that are chewing up scope stops? They are obviously not designed to do that.
The ironic thing in this case is that it's probably a western marketing guy that decided this rifle would sell better at 495 fps, not a Chinese engineer.
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I'm not underestimating the G's stopping on air can make, but I think you good folks underestimating the G's contact can make which is day and night greater and imo the source of all the scope problems. So stopping on air would have xxG's, stopping on the seal would have much more G's, and stopping metal to metal would have substantially more G's. The first not causing any issues, the second causing scope issues, the third destroying the gun.
I'm not saying the air is not slowing the piston down, I'm saying it isn't slowing it enough and the seal takes the remainder whatever that is left which no doubt varies drastically with different guns.
Btw, the seal compression thing of ~.007" I mentioned was tested using a fresh seal sanded down to ~.012" above the dovetail, assembled, shot once or twice, looked for metal to metal contact, sand some more, repeat. The point was to see how far I could sand the seal down w/o devastating metal to metal contact because the closer I get the better the gun works, not just power but it generates more pressure to slow the piston, which reduces impact. If btw the piston never impacted then I wouldn't see changes. The .007" was the last gun I tested which had a short stroke and large bore. A longer stroke and/or smaller bore no doubt needs more, like .010 which I use on other guns. Yes I understand the pressure can compress the seal too, but not like impact. I suppose enough pressure would push the seal back to expose the dovetail, but I think that's way beyond what it sees. The best I can imagine is air compressing it a couple thou and the remainder is impact. Or maybe the first half of compression takes xx force, and the last half takes many times that so the air does half but the brunt of impact does the rest. Or some variation the above theories. The same test but dry firing would be interesting, but then the piston is moving faster on impact so I suppose not much use.
The numbers were off on JohnK's deal with 1300 making 4k, but I hear what you're saying. I imagine the pressure is higher than 1300, I was thinking more like 2k, so 1500 to 2k of force. Still a lot and certainly capable of stopping the piston given enough distance, but, imo, the pressure spike to stop the piston has two problems; one is it happens rather late and I'm sure we can agree it needs some distance to stop the piston. Two is that peak pressure is a spike that is quickly bled off as the pellet moves. So maybe the pressure is there at the last mm, but gone just as quick allowing a good solid impact. This is what I was saying about how blocking the port will allow the pressure to stop the piston. Gloob dismissed this, but if enough pressure is only built in the last mm or so then the piston is probably pretty close to full speed either way. Yes slower blocked, but I don't believe it's significant. So I suppose just more speculation which is really all any of us have anyway, other than the testing I did which is open to interpretation.
And that bullet removal of yours Gloob; G's... The lighter faster hammer was higher G which is why it moved. Just like hitting a window with a BB at 300fps making 1ftlb energy can break it where a heavier force moving much slower like a hammer at that 1ftlb energy will never break it. Same with a softer object which disperses the G's, like hitting glass with a BB or pellet, the BB breaks the glass, the pellet, even with much greater energy splashes like a raindrop. The soft pellet was unable to apply what energy it had in a short enough time frame to generate enough G's to break the glass. The BB transferred it's energy in a tiny fraction of that time, which was enough G's to break the glass. My view is the same with the piston when comparing air to seal, both happen in a short span, but one is much more abrupt and can be felt and heard in the gun by blocking the port to prevent the seal from touching, or at least slamming.
And yes I know the impact of the piston typically barely moves a scope, assuming it's securely attached, so it can take many shots to even notice. Low power but high G like breaking glass. A BB doesn't move the glass much at all, but it breaks it. The piston impact doesn't move the scope much at all, but it can move it where a firearm with many times the recoil energy cannot, because the firearm never reached the minimum G's needed to get it to move.
I wonder if the Government would give me a million $ grant to study this in more detail?
I'm not saying the air is not slowing the piston down, I'm saying it isn't slowing it enough and the seal takes the remainder whatever that is left which no doubt varies drastically with different guns.
Btw, the seal compression thing of ~.007" I mentioned was tested using a fresh seal sanded down to ~.012" above the dovetail, assembled, shot once or twice, looked for metal to metal contact, sand some more, repeat. The point was to see how far I could sand the seal down w/o devastating metal to metal contact because the closer I get the better the gun works, not just power but it generates more pressure to slow the piston, which reduces impact. If btw the piston never impacted then I wouldn't see changes. The .007" was the last gun I tested which had a short stroke and large bore. A longer stroke and/or smaller bore no doubt needs more, like .010 which I use on other guns. Yes I understand the pressure can compress the seal too, but not like impact. I suppose enough pressure would push the seal back to expose the dovetail, but I think that's way beyond what it sees. The best I can imagine is air compressing it a couple thou and the remainder is impact. Or maybe the first half of compression takes xx force, and the last half takes many times that so the air does half but the brunt of impact does the rest. Or some variation the above theories. The same test but dry firing would be interesting, but then the piston is moving faster on impact so I suppose not much use.
The numbers were off on JohnK's deal with 1300 making 4k, but I hear what you're saying. I imagine the pressure is higher than 1300, I was thinking more like 2k, so 1500 to 2k of force. Still a lot and certainly capable of stopping the piston given enough distance, but, imo, the pressure spike to stop the piston has two problems; one is it happens rather late and I'm sure we can agree it needs some distance to stop the piston. Two is that peak pressure is a spike that is quickly bled off as the pellet moves. So maybe the pressure is there at the last mm, but gone just as quick allowing a good solid impact. This is what I was saying about how blocking the port will allow the pressure to stop the piston. Gloob dismissed this, but if enough pressure is only built in the last mm or so then the piston is probably pretty close to full speed either way. Yes slower blocked, but I don't believe it's significant. So I suppose just more speculation which is really all any of us have anyway, other than the testing I did which is open to interpretation.
And that bullet removal of yours Gloob; G's... The lighter faster hammer was higher G which is why it moved. Just like hitting a window with a BB at 300fps making 1ftlb energy can break it where a heavier force moving much slower like a hammer at that 1ftlb energy will never break it. Same with a softer object which disperses the G's, like hitting glass with a BB or pellet, the BB breaks the glass, the pellet, even with much greater energy splashes like a raindrop. The soft pellet was unable to apply what energy it had in a short enough time frame to generate enough G's to break the glass. The BB transferred it's energy in a tiny fraction of that time, which was enough G's to break the glass. My view is the same with the piston when comparing air to seal, both happen in a short span, but one is much more abrupt and can be felt and heard in the gun by blocking the port to prevent the seal from touching, or at least slamming.
And yes I know the impact of the piston typically barely moves a scope, assuming it's securely attached, so it can take many shots to even notice. Low power but high G like breaking glass. A BB doesn't move the glass much at all, but it breaks it. The piston impact doesn't move the scope much at all, but it can move it where a firearm with many times the recoil energy cannot, because the firearm never reached the minimum G's needed to get it to move.
I wonder if the Government would give me a million $ grant to study this in more detail?
jcwit
member
Some here have way to much time on their hands and should spend it target shooting or some other endeavor.
If you look at the pressures involved, it should be clear that the air presents a HUGE amount of force that the piston must overcome to hit the end of the chamber. What that means is that in practice, since the air exerts so much braking force on the piston, it's essentially immaterial whether or not it actually makes contact at the end of the stroke or stops before hitting it or rebounds slightly. By the time it gets close enough to make contact most, if not all of its velocity has been cancelled by the force of the pressure.
That's assuming a pellet of reasonable weight and fit in firing position. If there's no pellet, or a very loose and/or very light pellet then you don't get the pressure build and you could definitely get the piston to hit the front of the chamber at or near full velocity.
Yup, I screwed up the area calculation--good catch. I'll edit my post to reflect the correction.
The surface area of a piston with a 1" diameter is 0.785 square inches. 1300pounds per square inch on 0.785 square inches is 1,021 pounds of force.
Correct, 2,000 psi would exert about 1,570 pounds of force on a 1" diameter piston.
It needs some distance but it may not need very much at all depending on the force available. If you have 1,000 to 1600 lbs of force available then you can stop something weighing 3/4 of a pound very rapidly and in a very short distance. In the Cardew book, the piston travel trace shows that the piston travel reverses in half a millisecond or less and goes from full speed to full stop in about 0.05".
It's the fact that it stops so fast, and in such a short amount of time that generates so much acceleration. But that's what you'd expect from the amount of pressure that's opposing the piston (and therefore the amount of braking force on it) at that point.
If you can convince them to do so, I'd like to be first on the list to see the results!!!
I'd rather be at the range, but even when I can't be, I tend to spend my time thinking/working on shooting related stuff. Some people have lots of hobbies but I spend most of my spare time on just one.
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jcwit
member
Solved that by setting up a range in my garage.
But to each his own.
Thanks for all your research.
I've got an airgun range in my garage but this time of year in TX it's pretty miserable out there. I can stand in the house and shoot into the garage which is more comfortable, but if I do that for any significant amount of time, my expensive, cool, indoor air starts mixing with the free, hot garage air and that make me unhappy when my electric bill comes.
The "research" is just what I read for fun. It's nice that sometimes some of it turns out to be interesting to other folks too...
The "research" is just what I read for fun. It's nice that sometimes some of it turns out to be interesting to other folks too...
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