No not saying they have a long stoke, i understand they use a short stoke design, just saying that the mid length system has a softer kick than the 10.5 short stroke system, not a big difference or really even note worthy but its there.
There may be no formal definition of long stroke or short stroke, but here is mine.
I think that a long stroke piston is powered by bore pressure over all or most of the complete length of the operating cycle. Two that currently come to my mind are that of the M1 rifle and the FAL. The piston end of the op rod is contained in the gas cylinder for the entire cycle. THERE IS NO VENTING OR SHUT OFF AFTER THE PISTON BEGINS THE STROKE. The FAL has an adjustable vent at the beginning to regulate the pressure. The M1 has no vent.
As far as I know, most other current and successful designs are short stroke in one way or another. Either they have a separate piston that has a much shorter cycle than the carrier-locking lug assembly or they are vented or both.
The AK, H&K, and M16-M4 are vented shortly after the piston begins the cycle. In the case of the M16, it is the cylinder that moves and the piston is the bolt head, which does not move under gas pressure.
I always thought it was pretty simple in that long-stroke were generally indicated by the rod attached to the carrier and short-stroke were generally rods that strike, or push, the carrier. AK being long-stroke, FAL being short-stroke.
Someone earlier in the thread said that long-stroke was the piston moving a distance equal-to or greater-than the length of the case and short-stroke was the piston moving a distance less than the length of the case, which would go hand-in-hand with my definition as well.
This is due to the very large open area and clearances of the fit between the bolt carrier and receiver. The carrier just rides on two sheet metal rails in the sheet metal or milled receiver instead of a carrier in a cylindrical mostly closed up upper receiver. Apples don’t equal oranges.
I appreciate the design of the AK it’s a great gun (I own a few of them), it isn’t the same as an AR. It’s not quite as easy to change calibers, use good optics and lasers etc with an AK. With an AR I can go from .22LR, 5.56mm NATO, 6.8SPC, 6.5Grendel to .50 Beowulf in about 1 min. Not so with the AK.
That’s my working definition as well.
However, I’ll throw it out there that it doesn’t really matter.
The most meaningful difference between various systems is: how long the gas system is under pressure, or short impulse vs long impulse.
The most obvious manifestation of this is the barrel length from the gas port to the muzzle. ( sound familiar ?)
Once the bullet un-corks from the muzzle, the game is over, and whatever work needed to get done better have already been done.
A piston that has one inch of bbl after the gas block, has a lot of work to do in a very short period of time, a short impulse design.
The AR, seems to be one of the longest & softest impulse designs around.
Another example, the “long stroke” AK, will operate happily with the " gas tube" ( really just a piston guide ) removed.
The only powered travel is roughly .75", just when the piston is inside the gas block, before the radial gas ports are exposed.
The thing that really controls the " dwell time" is the barrel length ,velocity of the round and any gas storage effects the system may have.
This is what most suppressors and the KX3 do, increase dwell time and increase the overall impulse delivered to the piston.
The gas port diameter controls the “rise time” ( how quickly it ramps up ) and fall time of the pressure in the gas system.
I have seen this train of thought before what long stroke rifle has a long impulse? From what I can tell the ZB26,Bren,BAR,PKM & AK all vent the gas early IE short impulse. I am sure some where in the firearms world there has been a long impulse design but the above represents to the world what long stroke operating system are.
Not in my experience. There are issues with the trigger group if dirt gets in there (the AK).
My experience is the exact opposite. The AK can take a substantial amount of dirt and abuse, more so that most other systems. It can also remain functional with little if any maintainance.
I recall a ‘test’ done on a whim about 25-30 years ago with an AK. A fellow took about (IIRC) approximately 5-6 small-med marbles, dropped them into the receiver, insterted a mag, and proceeded to run the rifle through it paces w/o any problems. Removing the top cover after shooting several mags revealed that much of what was left of the marbles was small pieces. I can’t say the same results would be realized every time if this was done, but just the fact the receiver would accomodate the marbles is something.
I took fine sand and poured it into the side of the receiver. The AK stopped working. I had to open the receiver of an AR and pour sand inside to cause problems. Even then, the trigger just didn’t reset and could be pushed forward with a finger to reset.
Colt made a Long Stoke piston (Colt’s initial M16A2 concept gun of the late 60’s or early 70’s). We have a few here, the piston is part of the bolt carrier (well it links into it).
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The way it was explained to me:
On a Long Stroke, the piston is connected solidly to Bolt Carrier and moves the same length that the Bolt Carrier moves while it cycles the cartridge. The Long Stroke is a softer, less violent action because the BC is powered its entire length by combustion gas.
Basically, on a Short Stroke, the piston just touches up to the Bolt Carrier-no positive link, just bumps up against it. Once the gun is fired, the piston will get the BC moving rapidly rearward but the piston will stop at 3/4" or so. Once the piston stops, it isn’t pushing the BC anymore but the BC continues rearward under it own momentum to cycle the cartridge. The piston just “catapults” the BC back for the rest of its cycle. This is a more violent action because the piston has to be powered aggressively to insure the BC will cycle when the gun gets dirty, for different ammo strengths, etc.
Is this clear as mud? Is it correct?
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No, I’m not saying long stroke= long impulse.
To me, long or short stroke is how far the piston moves.
The impluse is what matters more.
Take the scar with a consistent placement of the gas block and piston.
The 10" barrel vs the 18" barrel.
Big difference in duration, but the total impulse and carrier velocity is roughly the same.
Which appears to have been the basis for the Korean K-2 rifle. The K-2 is a piston AR done right IMO.
I got it explained other way round.
Long stroke has piston travel all way with bolt back and forth, increasing moving mass and making rifle less stable. Example given - AK
Short stoke on other hand have piston and rod move only part way with bolt and then move forward at same time the bolt still travels backward. In this case overall momentum of moving masses is reduced (momentum of piston/rod moving forward is deducted from momentum of bolt/bolt carries moving forward as it has opposite vector of force direction). This makes rifle operate less violent and more stable. Matter of timing and moving masses. Example given - SVD
In AR world both short stroke and long stroke system add considerably less mass so it’s may have less impact here. But I never found short stroke system like in G36 or SL8 to be violent. In fact I was always amazed how soft shooting they are.
No, the gas pressure on the case does not move the bolt back, nor (as some assert) does the gas pressure inside the carrier push the bolt forward to aid in unlocking the bolt. As soon as the bullet clears the gas port, the gas pressure from case web all the way into the carrier is equalized. You need a pressure differential to push something. At this point, the pressure can only move the carrier rearward and the bullet forward
Ever seen carrier velocity graphs of suppressed versus unsuppressed or watched them under high speed video?
Cause if you have you would know your answer is incorrect.
nor (as some assert) does the gas pressure inside the carrier push the bolt forward to aid in unlocking the bolt. As soon as the bullet clears the gas port, the gas pressure from case web all the way into the carrier is equalized. You need a pressure differential to push something. At this point, the pressure can only move the carrier rearward and the bullet forward
Dude your understanding of the whole thing is just skewed…
In the AR, Case pressure does push the bolt back, about .002" until the locking lugs of the barrel extension stop the bolt, just like with a bolt action.
Regarding the bolt moving forward, You don’t need a pressure difference to push something, you need a force difference.
In the case of the AR, the equal pressure in the chamber and piston produce different forces because of different areas/ diameters.
The case has approximately .330" inside diameter.
The piston on the bolt has approximately .430" effective diameter.
This force difference does push the bolt forward and away from the extension locking lugs.
The reaction force pushes the carrier rearward.
By the time the carrier has moved far enough to start unlocking the bolt, the bullet has left the barrel and chamber pressure drops quickly.
Clint, I see that the thrust to the bolt face will take up any slack there may in the bolt lock up when the weapon is fired.
However the point I was making is, when the gas system is pressurized and the bolt carrier begins it’s motion, there is equal pressure in the case and in the carrier. As such, the pressure would not be pressing the bolt back, it is pushing the carrier.
Clint, you are correct when I spoke of the pressure not pushing the bolt forward, I did not take the difference in surface area into account. However, the brass case (note I say brass not steel) has also expanded under pressure to fill and seal the chamber, including the headspace and also grips the chamber walls, resisting any movement of the case, forward or aft. I do not know if this stiction is enough to overcome the force difference due to the diameter differences between the front of the bolt and the rear, but I do know that it’s enough to significantly reduce bolt thrust. P.O. Ackley wrote about this.
KevinB, if my understanding of the whole thing is skewed, perhaps you can explain how it really works.
Self loading weapons are designed to ensure the bullet has exited the muzzle and pressures have dropped to safe levels before the bolt unlocks. This is to ensure that the bolt isn’t trying to extract the fired round while it’s sticking to the chamber and to keep high speed, high pressure gases from damaging the weapon and shooter. By the time the bolt of the AR unlocks the gases have vented and pressures dropped dramatically. Still, they will be relatively equal on both sides of the piston and the jetting action of the ejecta mostly dissipated. (The ejecta does add significantly to recoil).
Suppressors change the equation, in particular by increasing dwell time and slowing the escape of gases at the muzzle.
I do have a couple of questions as I realize every design has it’s quirks-
-What source references to the bolt being pressed forward to aid in unlocking? Is this a theory or has it been measured and proven?
-What source references there is enough gas pressure on the face of the bolt, after it’s been unlocked, to aid in it’s rearward motion?
the bolt does not move forward, there is insufficient pressure in the carrier to do this, further the ar design relies on residual chamber pressure to ensure extraction. there is an ndia briefing which demonstrates this here: http://www.dtic.mil/ndia/2003smallarms/din.ppt#1
obviously, this happens over a very small period of time but chamber pressures throughout are always greater than carrier pressures.
in modern terms , the op rod is attached to the bolt carrier in a long stroke piston design; in a short stroke design the op rod is not.