Long Stroke vs Short Stroke

[sawdeanz]

Been seeing a lot about the new ARAK21 from faxton armory, and one of their big selling points is the attached, long stroke piston which is much closer to the original AK design then many of the other piston ARs. I find this odd considering the trend after the AK and Garand has been to short stroke pistons, presumably because they could bend like on the Garand, or because the weight and recoil were worse like on the AK.

So what's the deal? Is it mostly dependent on the gun? Does it offer any reliability increase over a short stroke piston?

In this contest I take long vs. short to mean long=rod connected to bolt and short = rod and bolt separate.

 

[eldon519]

I think long stroke is often regarded as a bit more reliable because you have more reciprocating mass, but then you also get more felt recoil from it. The short stroke recoil is a bit more spread out.

I could be wrong though.

 

[wally]

I thought long stoke vs. short stoke is based on how far the piston moves before the gas vents, not how long the rod/piston is or if there is an intermediate rod.

By analogy with engines, its long stroke if the diameter of the piston is less than the distance it moves before the gas vents.

By this definition AK is definitely short stoke as they'll run fine with the gas tube removed if you manage to keep your fingers out of it

The long stroke should be "softer" as the piston accelerates for a slightly longer time.

 

[briansmithwins]

Long stroke is usually defined by the piston moving more than 1 cartridge length. Short has the piston moving less than 1 length. Venting doesn't typically enter into it.

It's from the same set of definitions as long vs short recoil operation.

Long stroke is typically simpler and usually offers a higher recoiling mass as you can add the piston mass to the bolt carrier since they are usually one piece.

Short stroke is often implemented by using a light piston traveling at higher velocity that transfers energy to the bolt carrier but doesn't move with it for the entire cycle. The separation can also help prevent heat transfer.

For a modern implementation of a long stroke piston see the Tavor's mechanism. Most other modern rifles are using short stroke systems.

BSW

 

[barnbwt]

"I thought long stoke vs. short stoke is based on how far the piston moves before the gas vents, not how long the rod/piston is or if there is an intermediate rod."

I think both definitions are correct, but this one has become outmoded. Long stroke, in automotive piston applications, actually does have to do with the proportion of piston travel driven under pressure before venting, and I suspect the initial firearms nomenclature mirrored this.

But firearms design subsequently fell into essentially two camps, most clearly differentiated by whether or not the gas-impinged piston head was hard-linked to the bolt operating device. Since no one knows, or is expected to know, the displacement of a given piston before it vents, the earlier description isn't really that useful in conversation. But the prevailing description detailing piston attachment is very descriptive when it comes to discussing or comparing different designs. It provides a starting point that puts everything into two buckets, instead of a continuum of multiple variables (like we have for cartridges. Just imagine the .5" vs. .505" driving distance debates we could be having! What fun! )

Long Stroke:
-Generally more aggressive operation (this is more the cause of increased recoil than anything)
-Tend to be overgassed (the extra reciprocating mass can absorb the extra energy without speeding up so fast it overrides the magazine and jams) for marginal reliability boost
-Tend to be simpler (the parts count and receiver designs, anyway)
-Tend to be more easily tuned (more fixed variables)
-Tend to have smaller bolt & carrier contact surfaces in the receiver (the gas tube can act as a primary guide)
-Slower cyclic speed (can be a blessing or a curse, depending on requirements)

Short Stroke:
-Intermittent recoil impulse (piston returns separate from the carrier, which can counteract each others' impacts; people used to AKs hate the felt difference, and vice versa)
-Tend to be self-regulating (regardless its violence, the pressure impulse can only drive the piston so far before being completely cut off)
-Tend to have more complex receiver rails or bolt carriers (since the piston cannot guide the carrier)
-Higher parts count (usually an extra spring, but that's usually it)
-usually better ejection port access (no piston in the way once retracted)

I'm no expert on AR conversions, but to be honest the tubular bolt carrier design of the platform is non-ideal for either (but especially the short stroke since there are no small contact areas to control up/down pitch, only broad surfaces inside the receiver tube with lots of opportunity to exert friction). That's not to say it doesn't work, after all, the HK416 is highly regarded, but obviously it's taken a whole lot of time and energy for anyone to export the initial success of the AR180 into the tubular AR chassis (I honestly wonder if a modern re-visiting of the VZ52 annular piston in a world of non-corrosive powders might be in order; that gun had a cylindrical carrier after all, and was quite reliable if corrosion was kept at bay)

TCB

 

[wally]

Thanks for the clarification.

Long stroke is usually defined by the piston moving more than 1 cartridge length. Short has the piston moving less than 1 length. Venting doesn't typically enter into it.

Since the carrier has to move more than the cartridge length to unlock the bolt and pickup the next round, it would seem that short stroke implies an intermediate rod where long stroke would imply piston attached to the carrier.
So SKS would be short stroke, AK long stroke, I'd assumed the reverse since it looks like the SKS piston moves a bit more that its diameter and the AK a bit less before the gas vents.


Makes sense, I'd never seen an authoritative definition, so the analogy with engine terminology made more sense to me. I figured the manufacturer labeled it correctly in their specs, as was said, .5" piston with .4" travel vs .6" travel would not be all that obvious.

 

[Ar180shooter]

It has always been my understanding for long stroke vs. short stroke, in a long stroke system, the piston reciprocated with the bolt carrier for the entire length of the cycle, and is often integral with or attached to the bolt carrier (AK-47, Sig 550), while a short stroke reciprocated with the bolt carrier for a portion of the cycle, returning to rest before the bolt carrier fully cycles (SKS, VZ-58) or is returned to rest by the bolt carrier tapping it forward on the return stroke (FS 2000, SCAR).

 

[briansmithwins]

The FN SCAR and F2000 both use short stroke pistons that aren't spring loaded. Those pistons stay rearward until the returning bolt carrier taps them forward.

The M14 design is similar.

BSW

 

[Ar180shooter]

The FN SCAR and F2000 both use short stroke pistons that aren't spring loaded. Those pistons stay rearward until the returning bolt carrier taps them forward.

The M14 design is similar.

BSW

Interesting, didn't know that about the FS2000 and SCAR. I edited my original post to indicate this method. Same idea though, except they use the BCG to return forward rather than a spring. The main thing I was trying to get across was that in a short stroke, the piston only travels rearward for a portion of the cycle, and a long stroke travels with the bolt carrier for the entirety of the cycle.

 

[barnbwt]

You also have tappet guns, like the Carbine, which have the piston/tappet pick up speed before impacting the carrier (at least I thought that design gave the tappet a running start; I don't have one in front of me)

TCB

 

[Caliper_Mi]

So what's the deal? Is it mostly dependent on the gun? Does it offer any reliability increase over a short stroke piston?

In this contest I take long vs. short to mean long=rod connected to bolt and short = rod and bolt separate.

AK and M1 are long stroke, FAL and SKS are short stroke. All four are known for superb reliability. As long as the gas system is designed properly (gas port diameter, piston size, reciprocating mass, spring force, etc) either system will provide a reliable rifle.

Short stroke gas systems do divide the operating system into more parts. This could be seen as more failure points, or could be seen as easier to make parts and simpler service. A bent op-rod on a long stroke system means replacing the entire piston/op-rod/bolt carrier assembly where short stroke means just a new op-rod.

 

[briansmithwins]

Long stroke systems provide the trooper with one less part to lose, but, if the gas piston and op rod are above the bore, long stroke also means you can't reload the magazine with stripper clips, like the SKS and FAL were designed for.

You can use a long stroke system with a top mounted magazine such as the BREN gun does, but too mounted mags have their own problems. Since it's easy to run a belt of ammo across the top of the action many belt fed MGs use a long stroke piston.

On the FS2000 the space above the bore is occupied by the ejection tube and the gas system is offset to 10 o'clock. The SCAR has the gas system mounted at 12 o'clock and could have been made using a long stroke system. However, it has the gas block and piston mounted on the free floated barrel while the bolt carrier is on rails in the receiver.

BSW

 

[barnbwt]

"You can use a long stroke system with a top mounted magazine such as the BREN gun does, but too mounted mags have their own problems. Since it's easy to run a belt of ammo across the top of the action many belt fed MGs use a long stroke piston."

The BREN's piston is actually underneath the barrel. The way it gets around the cartridge hitting the op-rod, is that the rod is a massive steel block with a large slot cut down the middle. The bolt sits on top of it, and is so tall that it can reach up to strip rounds from the top-mounted magazine.

The mag was put up there (on the BREN as well as the Madsen, DP28, Lahti, and others) based on WWI trench-warfare experiences, which used a lot of prone fire. Belt-fed and strip fed systems of the day were terribly unreliable in that war, so box-fed LMGs were preferred afterwards, but boxes of useful capacity are too tall for prone-usage. Solution: put them up top or one the side.

A great many belt-feds actually use the inverted form of this architecture for their op-rods & bolts to this day, which is funny since it dates all the way back to the earliest belt-feds (ZB26 LMG I know for sure has it, but I think some of the older systems had it, too). The reason belt guns love the two-railed, super-massive carrier/piston setup is because it is heavy, and reciprocating mass is everything in belt gun reliability. Those guns practically have to be capable of ripping the links apart and keep cycling in order to ensure the belt won't bounce wrong and stop the gun at a bad time. And since the belt-feed levers usually have some pretty lousy leverage, that means the parts driving them need to be really heavy.

"Short stroke gas systems do divide the operating system into more parts. This could be seen as more failure points, or could be seen as easier to make parts and simpler service."
At least in the VZ, it makes the gun more modular, potentially. You can service the bolt/FCG area, or the gas system. The two are totally independent, and joined solely by the receiver rails. A modular AR-type platform taking advantage of this could have a gas system/barrel unit that slips off at the barrel nut, allowing for low-impact length and caliber* changes. With sights/optics that have programmable pre-set zeros (will probably be a thing soon if not already) you get the benefit of entire upper swaps without duplicating near as many parts (and optics!). Make the front of the magwell integral to the upper (as it should have always been) and you can now accommodate a huge range of cartridge lengths with nothing but a barrel unit, magazine, and bolt head swap.

TCB

*chamberings with common case heads, since bolt/carrier remain. "Low impact" meaning fewer parts swapped out, but also allows each barrel/gas system unit to be tuned to the same bolt/carrier/buffer scheme, so no fiddling with those upon switchout, either