How is velocity affected by Locked Breech, vs Delayed Blowback, vs Blowback?

[wbond]

How is muzzle velocity affected by Locked Breech, vs Delayed Blowback, vs Blowback?

For example, suppose you had three .380s, each with a 3" barrel.

One is locked breech. One is Delayed Blowback. One is Blowback.

I realize the action type affects the felt recoil.

However, does it affect the muzzle velocity?

 

[gazpacho]

I suspect that you could get greater variation in velocity between two guns of the same make, than the averages of different types. You may even find a greater variation bullet to bullet in the same gun.

 

[wbond]

To Gazpacho and Everyone Else:

It sounds like your vote is that it doesn't matter.

Anyone else want to contribute?

I don't know the answer myself, but it seems intuitive to me that a gun that keeps the slide closed and a tight gas seal until the bullet has exited should have more velocity. Seems that way to me, but I don't know. I'm just guessing. That's why I'd like some input from others.

Thanks

 

[Fletchette]

As far as muzzle velocity goes, very little. They are all different mechanical designs that accomplish the same goal, and therefore use very similar amounts of energy. If one were really, really persnickety, one could measure the amount of heat buildup in the frame/breech area to see which design was more efficient (which one had less mechanical friction). I doubt the variance would be greater than the variation in powder charge.

 

[Jim Watson]

There was a writer who shot a Ruger .22 with normal blowback action and with the bolt clamped shut. It gave slightly higher velocity running in blowback than with the bolt clamped. He had theories about the brass sealing better as it started to blow back but no proven reason.

Any well balanced semiauto is going to keep the breech closed until the bullet is well gone anyhow.

 

[mete]

Of course you could talk about the HK P7 . The gas retarded blowback reduces velocity as some of the gas goes into the cylinder instead of pushing the bullet. But then it has polygonal rifling which has less friction therefore gives higher velocity !!!

 

[Double Naught Spy]

Except for gas systems (as noted) where the gas system effectively increases the volume of space inside the system for gas to fill, hence slowing the build of pressure by some terribly small fractional amount, velocity will be unaffected as the bullet has left the barrel and is well down range before things like recoil and the cycling of the action even start.

 

[1911Tuner]

re:

DNS wrote:


>>as the bullet has left the barrel and is well down range before things like recoil and the cycling of the action even start.<<
**************

Nope. Not exactly accurate. While the bullet will be downrange before the breech opens...or at least it will if everything is working correctly...recoil begins at the instant that the bullet moves. Once the bullet is gone, recoil is over.

In a locked breech, recoil operated pistol like the Colt-Browning design, the slide starts its trip rearward while the bullet is heading for the muzzle. The barrel and slide are tied together for a short distance. The slide pulls the barrel rearward with it, and when the barrel reaches its linkdown point...roughly 1/10th inch...the link starts to draw the barrel down and away from the slide. The breech is open...which means that the bullet must be out of the picture before that point, or you get kabooms in your face.

With the locked breech, short recoil operated designs, the bullet has to be there and it has to move in order for the slide to cycle. If the bullet exits before the slide moves, the slide won't move. If the bullet doesn't move, the slide won't move. The blowback only requires that the bullet is there...and the slide will cycle, regardless of whether the bullet moves or not.


Cheers!

 

[usp9]

Any well balanced semiauto is going to keep the breech closed until the bullet is well gone anyhow.

+1

Splitting hairs here.



1911 What do you mean by;

Once the bullet is gone, recoil is over.

Recoil is momentum, and occurs after a bullet has left a barrel for all practical purposes. Recoil and muzzle flip occur after the bullet begins to move. Inertia is a large part of the physics here.



Some interesting photos of guns at the instant of firing;http://www.americanscientist.org/template/AssetDetail/assetid/48547/page/1?&print=yes#48792

 

[Vern Humphrey]

I did a fairly extensive test on this myself. The argument was that a light slide/gun would result in reduced velocity. I clamped heavy lead weights to the slide of a Ciener conversion kit and fired it over a chronograph, then repeated the experiment without the weights.

The result was a mean velocity of 1055 fps for the light (unweighted) slide and 1048 fps for the heavy (weighted) slide.

 

[1911Tuner]

Recoil

usp9,

Recoil is the equal and opposite reaction end Newton's Law. If there is no bullet, recoil doesn't occur, other than the impulse generated by the gasses
expanding into the atmosphere...which will barely nudge a 1911's slide without the recoil spring installed when fired with a blank round...and I had to use 10 grains of Bullseye to get that slight nudge.

Once the bullet is gone, recoil is over. Any further movement rearward of the gun or the slide after that is purely on momentum...momentum conserved while the bullet is in transit.

Load a case full of powder and poke a bit of toilet tissue over the charge to keep it in the case...load it, and fire it. No recoil. Block the barrel so that the bullet can't move even a tiny fraction of a fraction of an inch and fire it.
No recoil. The bullet has to be there, and it has to move, or recoil as we know it doesn't occur.

 

[ABBOBERG]

I have also done some work in this area, having designed and built two 9mm pistols with identical-length barrels (4.2" - same length as HKP7-series). The first gun has a gas-delay blowback system, with .0005" clearance between the two piston lands and the cylinder wall, and the second gun is a lock-breech design. After having fired multiple brands of ammunition, I have found an average of 5% more kinetic energy coming out of the locked-breech gun than the gas-delay gun. Slide travel is about .08" when the bullet leaves the barrel in both guns. When I compare the ballistics to the HKP7, the locked-breech gun delivers about 12% more kinetic energy than the P7. I believe that this due to the grooves that HK adds to the chamber to help "float" the case out of the chamber to improve reliability. This translates to 40-50 ft-lbs of loss using +P ammunition.

A locked-breech gun's chamber is no more "sealed" than any other type of gun mechanism. In fact, some locked-breech guns, such as the S&W sigma series and Kel-tec, have much larger chamber clearance than, say, a Kahr, resulting in an additional energy loss of about 4-7% over the tighter-chambered brands.

 

[wbond]

Blowbacks start opening immediately at ignition. Don't they?

I've read in more than one place somewhere that blowback guns start the slide moving and chamber opening at the moment of ignition. So the slide is moving back and the chamber opening before the bullet is gone.

Perhaps that is also true with locked breech, but with locked breech the slide opening is delayed. With blow back actions it starts opening immediately at ignition. This is why blow back guns are dirtier and need cleaning more often. More of the explosion comes back into the action area (less goes out the muzzle). At least that's what I read. I'm going to have to try and find what source I read that at.

So doesn't it seem likely that a blow back would give less velocity? If so, how much less?

I'm not referring to fancy gas piston operated delayed blowback here. I'm talking about simple blow back like a Bersa or PPK.

 

[1911Tuner]

Delay

Quote:

>Perhaps that is also true with locked breech, but with locked breech the slide opening is delayed.<
*******************

Nope. The slide starts moving immediately...or trying to. Nothing moves immediately. There's that thing about objects at rest tending to remain at rest...but the slide isn't delayed by anything except its own inertia and the recoil spring. The reason that the breech opening is delayed in the locked breech is because barrel and slide are moving backward as a unit.

Blowback delays breech opening via spring load and/or slide mass.

Locked breech adds the mass of the barrel until the bullet is gone.

They both function because of the same forces and technically by the same principle. The two just use different methods of delaying the breech opening...or, in this case... slide and barrel separation.

 

[ABBOBERG]

I was using the gas-delay example because straight blowback is not practical unless you have a very massive slide. The recoil will usually be too severe. I did shoot a few brands of ammo with a straight blowback set-up in 9mm using a sliding bolt bench rig with roughly the same mass as a normal slide used on a full-sized 9mm gun. The velocities were no different than when I had the bolt locked shut. One big difference, however, came up when using Cor-bon ammo. The sliding bolt was not able to keep the primer from blowing out, and this caused about a 5% reduction in bullet energy.

I think that the key reason for seeing no velocity difference in standard-pressure ammo comparing a locked/closed bolt to a sliding bolt is that with 4" barrels, the bullet is already gone before the case mouth clears the chamber, so the pressure work is already done.

 

[Double Naught Spy]

1911Tuner, yes, thanks for splitting hairs. I poorly chose my words. I should have pointed out that there is a very very tiny amount of movement before the bullet leaves the barrel, but otherwise, the small amount accounts for just about zilch in the velocity difference. The tiny rearward movement doesn't alter the ballistics significantly...then add what gazpacho said.

 

[Vern Humphrey]

the small amount accounts for just about zilch in the velocity difference.

The amount of movement doesn't matter. What matters is the impulse. When the bullet starts to move, the rest of the assembly (slide and frame) gets all the impulse it's going to get.

All of the effect of recoil on the bullet occurs in the first milisecond or so.

 

[grendelbane]

Regardless of the action type, the rearward velocity of the slide is going to diminish the forward velocity of the bullet. Nothing to lose any sleep over, though.

Now, if the slide is some how locked to the frame, (or the pistol is a single shot, like a Contender, there might be a measurable increase in velocity.

If you thrust your arm forward vigorously while you are firing, you will also increase the velocity of the bullet. Not by a significant amount, however.

The highest velocity would probably be attained by a shooter swinging his pistol on the end of a rope, like a sling. Not very much extra velocity is gained by this approach, accuracy is difficult, and there are safety issues.

I definitely don't recommend this practice, but it does work!

 

[gazpacho]

Last year I chronographed Federal 110gr PD 38 Special from a 2" Taurus and a 3" Ruger. This is one of their premium loads, so one would expect premium results. However, after 100 rounds through each firearm, each firearm exhibited a velocity variation of over 70 fps. Roughly, that is a 7% variation in velocity, regardless of firearm, from premium factory ammunition.

I realize that this example involves revolvers and not semiautomatic pistols, but look at Vern Humphrey's experiment with his Cenier. His experiment turned out a mean variation of 0.7%.

 

[JohnKSa]

Even with the breech welded shut, there's still recoil--it's just that the entire GUN begins recoiling immediately whereas in an autopistol, only the slide (or slide & barrel) start moving at first.

I think that it's really a non-issue. The bullet moves, something moves in recoil--either the slide (blowback), the slide & barrel (locked breech recoil operated), or the entire firearm (single shot). I don't think it makes much difference how much of the gun starts moving and when...the same amount of momentum is imparted to the gun regardless.

The fact is that until the bullet leaves the barrel, the breech is effectively sealed. There's not much being lost in the way of velocity regardless of the action type. If there's any difference, it's going to be very small.

I guess that THEORETICALLY, the less mass that moves initially, the lower the bullet velocity. i.e. the less initial resistance that the gun provides to recoil, the slower the bullet will go. But I don't think that it's going to be a practical difference.

 

[Vern Humphrey]

Even with the breech welded shut, there's still recoil--it's just that the entire GUN begins recoiling immediately whereas in an autopistol, only the slide (or slide & barrel) start moving at first.

Exactly.

The entire mass -- slide, frame, and the hand holding the gun -- all move under recoil. And that recoil impulse is transmitted when the bullet starts moving. There may be some "jet effect" of the gasses as the bullet clears the muzzle, but essentially the recoil impulse is all over in a millisecond. The movement of the slide, frame and hand occurs after the impulse is imparted, but it's all foreordained at the moment the bullet starts moving.

 

[shield20]

OK - here is a question along these lines...

Will a handgun with a shorter barrel, which typically means less velocity from the bullet, generate less recoil also? I.E. - is the recoil bullet-velocity based?

I was wondering this when thinking about compact pistols. The thought is that they usually generate more recoil re: full sized because they are lighter, but is that recoil somehow diminished 'cause they generate less bullet velocity due to their shorter barrel?

 

[1911Tuner]

Velocity

Shield...No. Recoil isn't dependent on barrel length. As Vern noted, most of it occurs at ignition...Impulse. Although the bullet dwell time in the barrel will add a bit of push after the initial impulse, the effect is minimal in comparison
to the sudden jolt when the round fires.

 

[shield20]

Thanks Tuner!

 

[JohnKSa]

Recoil isn't dependent on barrel length.

Overstated a bit, IMO.

Recoil is dependent on barrel length to roughly the same extent that bullet velocity is dependent on barrel length.

Recoil is dependent on the momentum of the ejecta which is a scaled product of the weight and velocity of the ejecta. The bullet makes up only part of the ejecta and the velocity of the ejecta due to the powder weight is pretty much independent of velocity. So, the increased velocity of the bullet due to the increased barrel length will mean more momentum to be transferred to the pistol.

But the barrel length has a relatively minimal effect on bullet velocity--what Tuner says about most of the velocity coming right at the beginning of movement (when the bullet pops out of the case) is, of course, right on target.

Then, throw in the complication that a longer barrel is a heavier barrel which will act to reduce recoil velocity and what you're left with is that felt recoil from a longer barrel with the same loading may actually be less...