How Does it Work: Long Stroke Gas Piston

The long stroke gas piston system was in its heyday about 50 years ago, and was popular in both rifles and machine guns. The principle is that the gas piston is fixed to the bolt carrier, and both cycle rearward for the full length of the cartridge upon firing. The system was used in such distinguished designs as the M1 Garand, Kalashnikov, Browning Automatic Rifle, and ZB/Bren light machine guns among others. By including the mass of the gas piston in the reciprocating parts, the long stroke system potentially carries more momentum when cycling, this improving extraction and feeding. This generally comes at the cost of increased perceived recoil, as the extra mass impacting the rear of the receiver at the end of travel is felt by the firer.


    • I would tentatively say long stroke with an angularly moving component in place of the usual linearly-moving piston, since the hinged swinging arm does NOT disconnect itself from the bolt at any moment as the gun cycles.

      • The Marlin-Rockwell version used as an aircraft MG by the U.S. Army in the 1920s substituted a gas coupling under the gas port which turned the gas stream through 90 degrees to blow directly back into the front of the piston, which was cupped like most later pistons.

        Since the piston travel was about twice the OAL of the cartridge, I’d say that yes, the M-R version at least qualified as a “long stroke” piston system.



  1. The term “distance” for gas operation should refer the “Gas effected” travel of gas piston within the gas cylinder. Since the gas comes from rather very high pressured section, its usage in a travel as driving the piston for a distance longer than the used round, would severely damage the gun and therefore, the taken gas should be exhausted after a short piston travel as gaining sufficient momentum to disengage the lock mechanism as leaving the cocking and ejecting missions to the simple blowback elements like gained momentum and residual gas pressure within the barrel. Through the light of this explanation, AK rifles should be considered as “Short Stroke”… Not long.

  2. The M-1 Carbine probably holds the record for the SHORTEST piston stroke in history, basically the piston acts as a hammer to start the operating rod moving.

    In rifles like the FAL, the stroke of the separate piston is also very short, the high pressure gas strikes the head of the piston, driving it rearwards a VERY short distance before it strikes the bolt carrier. At that point the energy is transferred to the carrier which then moves rearwards following the old M1V1=M2V2 rule of momentum, just like billiard balls but of greatly differing mass. Both the piston and the carrier are “elastic” in an “engineering materials” sense and suffer no measurable plastic deformation in the transfer.

    The venting of the gas in the FAL provides a different situation from that of the AK. In the FAL and L1A1 variants, it is the ‘dwell time’ that is altered, not the amount of impinging gas. Bleeding gas off whilst it is still being directed to the piston face means that a larger than theoretical optimum gas port cane be drilled in the barrel. This does NOT affect muzzle velocity in any significant way, because by the time the gas starts to act on the piston and then, when that moves, and the gas is vented, the bullet has left the muzzle.

    Comparing and contrasting the mechanics of the M-1 rifle gas system with that on the M-14 is interesting as well.

    • In my experience, the M1 and M14 have about the same felt recoil due to the way that the mass of the piston/operating rod assembly is brought to an abrupt stop when the rear end of the operating rod whacks the receiver at full recoil. No, the bolt doesn’t hit the rear receiver wall, it just feels that way.

      The two are about equally accurate in sniper/DMR workups, because while the longer stroke of the Garand’s gas piston gives a smoother action than the M14’s, which has about half the total travel, the bare forward three-fifths of barrel on the M14 gives something of a free-floating effect which is not seen on the original Garand.

      However it must be noted that in WW2, Korea and finally Vietnam, the M1903A3 sniper Springfield, even with a relatively inefficient low-powered telescopic sight, consistently outshot the M1 and M14/M21 self-loading rifles in the sniper role.

      There’s a reason that even today, most armies, special ops forces and police agencies which use designated marksmen give them bolt-action rifles rather than self-loaders. And it has nothing to do with nostalgia.



      • “which use designated marksmen give them bolt-action rifles rather than self-loaders.”
        Now I am totally confused, according to
        All designated marksman rifles in use today are semi-automatic, some are select fire. Sniper rifles are generally bolt action rifles, but can also be semi-automatic.
        So while there exist bolt-action repeating sniper rifle, what are examples of bolt-action repeating designated marksman rifles?

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