How Does it Work: Short Stroke Gas Piston

The short stroke gas piston operating system is common on modern rifles. It is defined as a gas piston which travels less than the distance of the bolt carrier (and is thus by definition not connected to the bolt carrier). This is in contrast to the long-stroke gas piston, which travels the full length of bolt carrier movement. The short stroke gas piston system was first made popular in the Soviet SVT-38/40 rifles, and was used in the Armalite AR-18, which formed the basis for many modern semiautomatic rifles.


  1. In some gas systems the piston violently kicks the bolt open after unlocking it and then returns to its position by spring tension. I wonder if my interpretation is wrong…

  2. The short-stroke system I’m most familiar with is the one in the M1 Carbine. Which is *really* short stroke, if you only count the movement of the tappet. Though the one-piece operating rod appears to move the full length of the bolt carrier.

    • In my opinion what in English is called “tappet operation” is a variant of short stroke where the “piston” did not push directly the bolt carrier but an operating arm that moves the action.

      • Yes, that is so. In some cunning design solutions the carrier has an extra extension forward (may be combined with charging handle), which is impacted directly by tappet without added operating rod. It is great system which has one potential shortcoming – it makes access to piston difficult for cleaning. That may not be a problem if propellant burns clean. Or, after some number of rounds it can by cleaned by solvent.

      • Sa80 “tappet” I.e. Gas rod pushes on bolt carrier, you have gas… plug, and piston; plug is what gets the gas from barrel (adjustable) piston is sleeve thing, over tappet. Tappet goes into carrier, lip hits it; causes carrier movement, rod is under its own spring pressure.

    • That is the inertial system. The entire weapon and the user’s shoulder recoil. The bolt unlocks as the rearward motion stops. When the user forces himself back into position the bolt opens under principle of static inertia (or simply put, the weapon moved back into battery faster than the bolt spring can shove the bolt forward). The shotgun shell is ejected. Finally, the bolt spring gets enough tension force from the initial compression to kick the bolt back into battery. I could be wrong.

        • This system requires at beginning carrier’s movement forward (to compress heavy driving spring). As it is cleverly arranged in Benelli shotgun, it would be difficult to apply for bolt carrier on typical rifle. On rifles when action is closed and locked, the carrier is in contact with barrel extension or trunnion; it cannot move any more forward.

        • Winchester Model 50 shotgun which partly being a design of famous Carbine Willams, may be classified as;

          – Recoil Operated since, having a floating chamber inside the fixed barrel recoiling with the locked bolt during the discharge for a distance to disconnect from that floating unit,

          – Inertia operated since, the delocking occurs by an inertia block connected at rear of the bolt into a cammed slot of the same as surviving its rearward travel after the bolt stops,

          – Gas operated since, the floating chamber might be accepted as a gas piston inside the fixed barrel sharing the same longitudinal axis…

          But it is considered by most as “Inertia Operated”, meaning no connection with current Benelli operation since delocking occurs by the force of direct momentum coming from recoiling motion. At Benelli, momentum first collected in an energy accumulator formed like a heavy spring and delocking is accomplished through its reflecting stage. First is one, second is two way ticket travel.

          By the way, in the video Carlos Villaroel pointed out, is a faulty image in which the motion of shell drop lever as rotating upward in the front, by the force of hammer spring during cocking and ejecting which can not be true since the rear of magazine latch (Not shown) prevents its upward movement until the rim of released shell reaching its rearmost location. İMHO.

  3. As first Ian is showing vz.58 rifle. This is really basic case of short-stroke piston application. Piston movement is only about 18mm (0.7″) upon end of which contents of gas block is vented into atmosphere. This is enough to impart momentum into bolt carrier.

    It might be of interest that we were not, under any circumstances, allowed to use for cleaning block’s inside an abrasive material such as scotch-brite. Apparently the concern was that by time the bore of gas block could become enlarged and endanger performance. However, soldiers in their inventiveness always found some ‘spare’ door handle at the end of which wrapped a swab of fabric. That created potential for material of handle to score inside of gas block. European door handles are different that those in N/A; they resemble a crank rather than a knob.

  4. One interesting detail related to mechanics of two or more bodies transferring momentum/ impact (such as suspended balls) and their mass. In order to transfer all of momentum available in impacting mass, the impacted one should be of identical mass. This is in practise hard to achieve and as a result there is some loss of momentum involved. It results in lesser mass body (typically piston) to bounce forward.

  5. Ok, so a short stroke piston arangment is out for pistols but what about a low profile gas tappet? That could be lower mass, right?

    • Who says short, or even long-stroke gas operated pistols are out? There is a bunch of so called pistols which are in fact ultra-short carbines with ‘brace’. Is that what you mean? If so, you can see that their operating system is typically gas piston.

  6. Short stroke gas operation is a kind of auto loading system which the imparted gas from the barrel is used only to delock the bolt from barrel and nearly no other function in to cock, eject and reloading stages leaving those to the simple blowback elements as remaining gas and gained momentum. İts working gap includes only the necessary distance to disconnect the bolt from the barrel and since the success to be warranted, the needed gas is taken from the barrel, at a place where the gas pressure nearly at its best. The used gas is exhausted in the most but İn some samples like M1 Carbine is contained within the gas vent channel and since this device being too little in mass, the operating rod through which the stroke is transmitted to the bolt, is constructed within a respectable mass to obtain a reasonable backward speed. IMHO

  7. Most of these pistols have no half cock notch in the hammer and since their way of loading nearly oblige users to carry with the chamber loaded, manualy decocking and cocking afterwards needs overcare against thumb slippage which may cause a catastrophe.

Leave a Reply

Your email address will not be published.