What is the difference between the long and short piston system?
I believe the AK uses a long stroke system! And i’ve read and heard about how reliable the AK is as in durability. So, why wouldn’t some of these company that make gas piston AR’s use the long stroke system instead of short stroke system?[?]
How much stroke the bolt carrier needs to move to unlock the lugs is part of the engineer’s concern. Once the lugs are unlocked, the case is driving the bolt carrier group back due to residual gas pressure in the bore.
You don’t just get to pick one over the other because somebody else designed a different firearm with it. So long stroke or short stroke isn’t so much a choice just because the AK uses it.
A lot of AK durability has to do with the goof proof magazine and simple design, not because it has one type of gas action vs. another. There are plenty of good designs out there that use either, the total execution is what counts.
Gas piston AR’s are a custom modification to the operating system. It’s not a fact they run more reliably, nor is it a fact that the DI system is less reliable when properly maintained. In fact, a good reading on the forums will show a lot of new AK owners with serious stoppages and failure to feed problems. Any firearm can have reliability issues - what the AK is known for is durability, a different concept. It means it can take a higher level of mistreatment and still function - not that it has a better ability to feed bad ammo or eat dirt. What is interesting is that a number of photos are starting to surface showing old M16’s in use in Vietnam- bad handguards, no anodizing, and still shooting.
The AK and M16 didn’t start out from the gate very evenly, but the race is still on.
The short stroke system has less reciprocating mass, that is centered therefore there is possibly less wear on the buffer and the rear of the bolt, due to less force being exposed to it.
Theoretically it’s also supposed to me marginally more accurate than a long stroke due to it being centered.
I prefer the short stroke system.
If I’m wrong I shall eat crow. My only experience with the long stroke system is the AK and FAL systems.
I’ve heard of this company before. Is there any reason why they would choose a long stroke over short?
They make a short stroke piston as well. LINK
The FN-FAL is a short stroke ,tilting bolt automatic rifle SKS is a similar style system.
Your wrong the FAL is a short stroke system.
Just FYI, there is another company that produces a long stroke gas piston kit., TNW Firearms (Not familiar w/them).
If I am not mistaken, the terminology has to do with the distance the piston travels.
A long stroke piston travels the length (or more) of the cartridge.
A sort stroke piston travels less than the length of the cartridge and usually hits/pushes etc some intermediary component that then moves.
The AK piston being directly linked to the bolt carrier travels the full length of said carriers travel.
The piston in say a M1A only travels a small distance, and imparts its energy to an intermediary op rod that then moves the bolt.
The short stroke is vastly more common than the long stroke from what I have seen.
As to the durability/reliability legend of the AK. It is the entire system that counts, not just the method of reciprocating the action. I have not seen any testing to support this but I hypothesize that AK’s being fed out of extremely poorly manufactured magazines would not live up to the AK legend. 
Upon re-evaluation, I shall eat crow. The FAL is short stroke.
Which may or may not be why it’s so accurate, compared to long stroke guns I’ve shot.
How much stroke the bolt carrier needs to move to unlock the lugs is part of the engineer’s concern. Once the lugs are unlocked, the case is driving the bolt carrier group back due to residual gas pressure in the bore.
It is the moving mass of the bolt carrier that drives the bolt carrier to the rear. Momentum. Newton’s First Law.
I thought that at one time. What moves the bolt carrier on a .22 blow back, or the slide on a semi auto pistol? Gas pressure in the case pushing back against the bolt face.
Roller locked bolts on HK’s? Case forcing the bolt back. There’s no operating rod.
I believe DI just opens the bolt. Case pressure pushes the bolt back, rotating the cam pin and forcing the head against the upper track, where the TM states to “lube generously.”
If I had two AR’s, I could demonstrate by connecting the gas tube of one to the other, observing whether it would cycle the bolt on the non fired gun, extracting, cocking, ejecting, and reloading. That would eliminate the affect of gas in the barrel.
I don’t even think it would eject.
Interesting test.
Do it.
A few things on this thread so far:
1- Long stroke versus short stroke definitions are vague at best. I an still waiting on an actual firearm engineer to weigh in on what the working definitions are. More important than how far the piston travels is how much reciprocating mass the operating system requires. Any mass outside the direct bore path will cause the gun to bounce more than one that has all mass within the recoil path. Any mass forward or behind the breech sealing device will cause the gun to bounce more than one with a shorter overall length. Any force exerted on the bolt that is outside the recoil path will cause more bounce when compared to an identical operating system with more centered operative force.
2- Much of the AKs reliable operation is due to loose fit between operating parts and robust magazines.
3- Rearward travel of the bolt and bolt carrier group in a DI AR are absolutely linked to gas from the propellant. Gas volume and duration must be properly balanced with the action spring and buffer weight to ensure proper function. Gas volume is determined by the size of the gas port, which is why different length gas systems require different gas port sizes. Short-stroking guns are the proof of this. Short stroking is fixed by opening up the gas port. This is why Bushmasters have large gas ports- to work with cheap, lower powered ammo, and why some Colts and LMTs don’t reliably work with said el cheapo ammo.
Don’t let different operating systems confuse your understanding of the DI operating system. Blow-back operation is not the same as Recoil Operation, is not the same as gas-tappet operation, is not the same as DI operation.
TROMIX, I think, did a 'siamese M16 with just that set up. They had reversed AR’s (back to back) with the gas system of the first feeding the second and vice-versa.
… Have to find a pic…
Pictures found …
Tromix Projects
Yes, saw those pics. The principal is the same as a dual turbo installation on a V engine, it helps balance the gas input/output so the engine banks share a similar load.
I imagine the crossed up gas inputs help regulate the full auto actions by synchronizing their reciprocal firing. It forces them to take turns.
That would also introduce the concept that pressurizing the bolt carrier is what propels it enough to cycle the action. Which means I could be completely wrong.
It would be nice if an actual engineer could chime in with pressure graphs and explain it all.
I hate to be the one to tell you, but you are completely wrong. See my above post.
Due to a severe derail of the thread, I moved Tirod’s last post into a new thread. If anyone wishes to continue the education of DI, please head over to:
https://www.m4carbine.net/showthread.php?t=53043
the noticeable difference will be in the felt recoil. The long stoke piston will be softer as the short stroke will be more sharp.
LWRC 10.5 and 14.5 Piston uppers for reference.
I beg to differ. By design the AK can “ead dirt” in better fashion than the M16.
So you are saying that LWRC 10.5" uses a short stroke and an LWRC 14.5" uses a long stroke. You are mistaken, LWRC uses only short stroke. The 14.5" uses their mid-length system which amounts to a longer intermediate rod, but it is still a short stroke.