Advantage Of Short-Stroke Actions?

[MistWolf]

I don't think we came any closer to settling the dispute about whether the AR-15 "piston inside the BCG" really constituted a short stroke piston or was simply a variation of Ljungman's direct impingement system

While we're at it, we can examine if the AK is simply a variation of the Ljungman DI system. After all, the gas block is nothing but a short gas tube and the piston is nothing more than a lengthened section of the carrier on which the gas impinges on. It's easy to see the Ljungman is actually a long stroke system. The cup (or piston if you prefer) travels the full distance as the carrier, just as it does with the AK

 

[MistWolf]

Actually, by the time the gas gets to the expansion chamber in the bolt carrier, the pressure in the chamber is pretty close to the pressure in the carrier

Before the pressure in the expansion chamber reaches the same pressure as the chamber, the action spring starts compressing and the movement of the carrier separates the gas key from the gas tube and cuts the flow of gas to the expansion chamber

 

[cdb1]

I have one AR and don't care what type of system it has, it works so I'm good to go. I do like to learn new things though and have read every post in this thread. MistWolf makes the most sense to me.

 

[Prijador]

Huh.
A couple of folks tried to answer the OP's question.

And then a lot of words were spilled about how an AR system works (and even how it is really the ultimate short-stroke piston system), and how no, that reason can't possibly be why short-stroke* is used.

What I would like to see is more answers about why "short-stroke" systems are so commonly used by firearms designers these days.

*Short-stroke - meaning not "direct gas", Stoner system, DI, or whatever you call the system on the typical AR.

 

[briansmithwins]

Short stroke pistons let you decouple the barrel from the bolt carrier. On the FN SCAR and CZ 805 the barrel is free floated from the receiver and the piston stops contacting the bolt carrier after it moves a short distance. I assume that FN and CZ tested both long stroke and short stroke piston alternative designs before settling on the short stroke design.

Using a short stroke piston also lets the designer get away with using a shorter piston guide tube, which will save weight.

Short stroke pistons are going to be more complex, with more parts to keep track of and possibly lose. It isn't like long stroke pistons have no advantages, after all.

BSW

 

[lysanderxiii]

Before the pressure in the expansion chamber reaches the same pressure as the chamber, the action spring starts compressing and the movement of the carrier separates the gas key from the gas tube and cuts the flow of gas to the expansion chamber

The chamber (case mouth), barrel (at the port), front sight base (front of the gas tube), and expansion chamber have been instrumented in several tests of the M16 and M4 to measure pressure. The entire system has also been modeled mathematically. There is quite a bit of empirical and modeling data about the pressures, velocities, and positions of all the parts of an M16/M4 with respect to time.

In both models and experiments, the pressure in the expansion chamber maxs out at around 2500 psi, about 1.5 to 2 milliseconds after primer ignition, which is the same pressure in the chamber at that time, plus or minus 200 to 300 psi. At about 1.5 to 2 ms after primer ignition, the bolt carrier has moved to the rear only .030 inch.

If you look at the bolt movement versus carrier movement, the gas rings on the bolt uncover the vent ports before the carrier key disengages the gas tube, by about a tenth of an inch.

 

[lysanderxiii]

One of several reports

 

[stoky]

Either way, they both go ....bang.

& blow & squeeze & suck

First time I shot an HK (after thousands of rounds through M16s and M4s), and I stripped the HK to clean it, I thought "What the...? Where is all the grime???

It's burned onto the cases, about twenty yards downrange to the right.

 

[barnbwt]

What I would like to see is more answers about why "short-stroke" systems are so commonly used by firearms designers these days.

They best isolate the hot/dirty aspects of powering a gun via combustion products from the areas of the gun that contain precision mechanisms. That's basically it. Long stroke invariably routes gas piston blow-by back into the receiver via the gas piston guide tube, DI by necessity vents a lot of gas right into the operating parts. Forget dirt, gassing of the shooter by itself is very much intrinsic to these designs and is a very real detriment (especially for compact SBRs or suppressed jobs)

Secondarily, for 'assault rifle' type arms (as opposed to heavy machineguns or handguns) the short stroke is well suited for reducing the reciprocating mass of the operating parts, compared to other action types (the exception being DI, which has other drawbacks mentioned already). Since these rifles aim to be as light as possible, the reduced weight and reciprocation are appreciated, as is the fact that short stroke gas systems tend to be a good deal shorter than long stroke ones (IIRC, this has to do with fouling, since long stroke gas sticks around longer before venting, which would result in more fouling in the gas system & blown back into the receiver if tapped from nearer to the chamber)

Thirdly, Short Stroke arguably makes for easier modular design, and reconfiguration, to different barrel lengths, silencers, and chamberings. Granted, adjustable gas blocks and the plethora of gas tube lengths that now exist give similar flexibility to long stroke and especially DI systems, but DI was not the choice for very short barrel systems until quite recently, and long-stroke requires reconfiguration of the carrier to swap gas system lengths. While similar redesign has been effected with different piston lengths & carrier modifications in the AK and of course the AR15, it really was slick how the G36 system could do a handful of different barrel lengths by simply changing the barrel and op-rod parts. Even the AR hasn't quite been able to approximate that level of flexibility, since my understanding is the buffer must still be re-tuned for best reliability when considering a rifle vs. SBR length gas system (but that is more a function of the AR carrier being rather light to start with)

TCB

 

[barnbwt]

Short stroke pistons let you decouple the barrel from the bolt carrier.

This, too; SS is way easier to free float, since some misalignment is allowed between the piston/op-rod axis and the carrier, the system is less constrained over the barrel (it's still possible to free float AKs and of course ARs, however). AR's actually aren't nearly as free floated because of gas-key fit, but luckily that part is fairly flexible compared to a piston rod so it can't carry significant force out to/from the barrel.

Short stroke pistons are going to be more complex, with more parts to keep track of and possibly lose. It isn't like long stroke pistons have no advantages, after all.

Granted I'm biased, but I'd also argue that SS are easier to service, since you don't have to tear the whole gun apart at the same time (bolt system and gas system). It's also a lot easier to swap a VZ58 piston than an AK's. Long strokes are still king where inertia matters, though, which is to say belt-fed systems, and that's not nothing.

If you look at the bolt movement versus carrier movement, the gas rings on the bolt uncover the vent ports before the carrier key disengages the gas tube, by about a tenth of an inch.

Good to know; I'd suspected the carrier vented before the bolt was unlocked, if for no other reason than to reduce the gas sprayed out the gas tube more-or-less straight back at the shooter when the key separates; I understand the MAS and AG-42 are rather unpleasant to shoot without glasses because of the gas released from the tube when the carrier blows back.

TCB

 

[Prijador]

I appreciate your responses, barnbwt, thank you.

(and thanks to stoky, for reminding me where the brass from an HK ends up - I remember joking about how, if two bad guys were coming at you from different directions, you might could take 'em both out at once if they were lined up correctly - one with the bullet, and one with the brass!)

...and lysanderxiii posted a BRL report, wow, that takes me back - I actually worked there, a looooong time ago...

 

[zaitcev]

So I don't know where that leaves you with short stroke. I honestly don't know the advantages and disadvantages it has, or why it's considered "the best choice" by the people actually out to win contracts, while DI and long-stroke are exclusively relegated to ARs and AK knockoffs. Anyone know why?

As a somewhat educated guess, advantages of a short-stroke piston are:

- Can be made less sensitive to variations in ammunition. This was first put into practice by "Carbine" Williams, when he invented a tappet gas system. That said, most modern rifles use a gas regulator, so this advantage is not fully utilized.

- Can be easier designed for field-stripping. I would say, Russians were pioneers of it. Tokarev's 3-part short-stroke piston was widely copied, in particular by Germans in G-43. Although, of course, everything can be screwed up. Look how much easier it is to disassemble AK than SKS. One way or the other, a long-stroke system has to be designed so that a long assembly were possible to extract from a receiver, and the short-stroke system does not.

- As was discussed extensively in this thread, a smaller reciprocating mass helps recoil mitigation for full auto, if it's a design objective at all, of course. Note, by the way, that machine guns tend to have long-stroke systems in part because there's no need to aim their bursts as precisely.

- Although it is no longer a consideration, a short-stroke piston does not conflict with loading by stripper clips (well, conflicts less - just recall how much effort John Garand spent on designing the machine that bent oprods). My recollection is somewhat vague, but I think it was a factor in the design of FAL, which inherited its gas system from SAFN, which was strip-loaded (IIRC).

- Finally, a short-stroke system offers a significant advantage in accuracy when shooting in single shots. I'm sorry to say, this was always a bit of a mystery to me, because the bullet exits the barrel long before either long- or short- stroke piston starts to move. However, it's pretty much a fact. Memoirs of various people all agree that it was a major factor why Dragunov selected it for SVD.

 

[Mosin Bubba]

Huh.
A couple of folks tried to answer the OP's question.

And then a lot of words were spilled about how an AR system works (and even how it is really the ultimate short-stroke piston system), and how no, that reason can't possibly be why short-stroke* is used.

What I would like to see is more answers about why "short-stroke" systems are so commonly used by firearms designers these days.

*Short-stroke - meaning not "direct gas", Stoner system, DI, or whatever you call the system on the typical AR.

Pretty much this.

I didn't know I was re-opening such a fresh can of worms, but the whole AR DI thing always seems to crop up whenever semi-auto actions get discussed. I figured calling it Stoner gas would be enough to avoid that, but apparently it wasn't.

If you say that an AR uses DI, then people go "well actually, they use a piston." If you say that an AR uses a piston, then people go "well actually, nearly all of them use DI."

 

[Mosin Bubba]

Then you have the interesting conundrum that always comes from these discussions, of balanced actions, which involve components shifting opposite the carrier to counteract the jostling of operation. The most interesting of these (practically all Russian) designs was this one, which opposes carrier inertia with the other heaviest component in the gun; the barrel:

I'd think with this arrangement, the only recoil delivered to the shooter would be from the bullet/powder gasses, and with a small bore bullet & muzzle brake/silencer, not even that. This type of action has long-stroke or DI as a huge advantage vs. short stroke, since the masses of the two inertially-balanced halves stays the same throughout the cycle. It'd be cool to replace that long, heavy piston & carrier nose with two opposing gas keys that come up to the same stationary gas tube.

TCB

Now that is slick.

 

[briansmithwins]

Secondarily, for 'assault rifle' type arms (as opposed to heavy machineguns or handguns) the short stroke is well suited for reducing the reciprocating mass of the operating parts, compared to other action types (the exception being DI, which has other drawbacks mentioned already). Since these rifles aim to be as light as possible, the reduced weight and reciprocation are appreciated, as is the fact that short stroke gas systems tend to be a good deal shorter than long stroke ones (IIRC, this has to do with fouling, since long stroke gas sticks around longer before venting, which would result in more fouling in the gas system & blown back into the receiver if tapped from nearer to the chamber)
TCB

Not sure about the short stroke gas being shorter than long stroke systems. The gas piston can be shorter in a short stroke system, to be sure, but there has to be an operating rod or extension of the bolt carrier to transfer energy to the bolt carrier. The minimum OAL of the gas system and recoiling parts would be dictated by the gas port location and cartridge length, gas system type doesn't really enter into it.

Examples:
The gas port of the Tavor (long stroke) is located very close to the chamber, leading to a shorter gas piston/BCG extension, while the FN SCAR (short stroke) has a gas port much nearer the muzzle. FN used a short gas piston but the BCG has a very long extension on it. The CZ 805 (short stroke) uses a 3rd method, gas piston with a long tail that contacts the short BCG.

BSW

 

[barnbwt]

Tavor is rather notorious for gas blowby into the receiver & shooter's face, however, obviously made worse by the shorter gas system in long stroke format. AKs get a bit grimy after lots of rounds, I can only imagine it would be worse with a gas system as short as a VZ58.

TCB

 

[lysanderxiii]

- Can be made less sensitive to variations in ammunition. This was first put into practice by "Carbine" Williams, when he invented a tappet gas system. That said, most modern rifles use a gas regulator, so this advantage is not fully utilized.

The gas system in the M1 Carbine is probably one of the most sensitive gas systems in existence. The short piston travel (about 1/8 inch) requires a very high port pressure and is very sensitive to operating rod drag during the initial kick from the piston.

- As was discussed extensively in this thread, a smaller reciprocating mass helps recoil mitigation for full auto, if it's a design objective at all, of course. Note, by the way, that machine guns tend to have long-stroke systems in part because there's no need to aim their bursts as precisely.

You have it backwards, the heavier the mass the lower the felt recoil, provided you allow enough travel for the mass to stop under action of the spring alone. If the mass bottoms out on the receiver you will feel a sharp impact of the mass.

Modern machine guns have lighter reciprocating masses in order to increase the rate of fire. And, that is bought by an increase in energy imparted to the other parts of the gun.

As noted earlier, long stroke pistons are favored by MGs so the feeding of the belt is a long continuous pull, rather than a sharp snatch.

- Although it is no longer a consideration, a short-stroke piston does not conflict with loading by stripper clips (well, conflicts less - just recall how much effort John Garand spent on designing the machine that bent oprods). My recollection is somewhat vague, but I think it was a factor in the design of FAL, which inherited its gas system from SAFN, which was strip-loaded (IIRC).

The M1 op-rod was bent to clear the enlarged diameter of the .30-06 barrel at the breech-end while keeping the muzzle end the same. The .276 version had a straight op-rod.

- Finally, a short-stroke system offers a significant advantage in accuracy when shooting in single shots. I'm sorry to say, this was always a bit of a mystery to me, because the bullet exits the barrel long before either long- or short- stroke piston starts to move. However, it's pretty much a fact. Memoirs of various people all agree that it was a major factor why Dragunov selected it for SVD.

The barrel moves as the bullet travels down its length, the question is: is the motion the same with each shot? The fewer external forces on the barrel the more likely the motion is repeatable. With a short stroke piston as found in the AR-18 (assuming there are no loads on the hand guard), the only external load on the barrel is the weight of the operating rod/cylinder (remember, the piston is on the front sight base).