The purpose of teh vent holes in the AKM gas piston is actually for the removal of CARBON that the piston head scrapes off the inside of the gas tube. As the piston runs back and forth, it also scours the inside of the gas tube. Once the piston passeds the holes, some of teh gas pressure then seeks it's easiest path out, and escapes through those holes -- carrying carbon fouling with it. This means that Private Ivan doesn't need to scrape the inside of his gas tube, even when using low-grade cruddy ammo. Any additional fouling (such as excess oil) used on teh piston or gas tube will get blown out as well. This has the effect of keeping the gun running under heavy fouling, without significantly changing the cycle time.
The contrasting view that an AKM is a "short stroke" system becuase of thge length of time it is under pressure ignores the fact that the AK47 piston spends MORE time under gas pressure than the M1 Garand due to the positions of the gas ports. Or that the AK47 and AKM gas tubes (interchangeable) somehow change the entire operating system from long stroke to short stroke.
Long stroke is, and has always been, defined as the gas piston traveling the full length of teh rewquired operating stroke. It DOES NOT need to be under full pressure to do so -- and in fact, NO gas piston, not even the M1 Garand, is under full gas pressure for the full stroke of the operating cycle. If they were, the gun would suffer a complete case rupture on each and every round fired, as the BARREL is under full gas pressure until the bullet leaves -- at which point BOTH the barrel and gas system vent to ambient pressure together. The gas piston starts out under gas pressure, but finishes it's cycle via INERTIA.
This yields a long, relatively smooth cycle. Doesn't matter if the piston is attached -- if it is pushing the bolt group the whole way, the whole mass is moving together with the relatively high mass smoothing out the cycle.
Short stroke systems impart a short, sharp smack to the bolt group. It can involve a conventional gas piston that hits a tappet (operating rod) that may or may not be attached to the bolt carrier. For example, the SKS carbine. Or, it may have a short piston like the M1 Carbine tappet. Or, it may have a conventional length gas piston that has a stop that prevents it from cycling the full length of teh bolt group's minimum operating cycle. The piston must impart enough energy via IMPACT, noyt inertia, to carry the whole bolt group through cycle.
It produces a sharp, short, cycle. . . but (in the case of systems without a full length gas piston, like the M1 Carbine), involves less mass cycling -- which can reduce felt recoil or weapon disturbance.
There IS a difference in how these two systems function, and the length of stroke is part of that. It is the physics of inertia, not gas pressure time, that defines short stroke versus long stroke. The definitions AS USED BY ORDNANCE DESIGNERS have been clearly set for over half a century. It was clearly understood that a long-stroke gas system moved the necessary operating stroke, and teh short stroke gas system did not, and that was the difference.
Or at least it was before Internet Experts decided to re-write the definitions on Wikipedia instead of using the actual textbooks on the subject that have stood for decades. But then, given the price and rariety of such basic texts as "Principles of Firearms" by Major Charles Balleisen (used as a primary source by Britannica, used as a textbook for the US Army Ordnance Corps, and later rewritten into an Army Technical Manual) and the five volume "The Machine Gun" series by Colonel George Chinn (written for the US Navy Bureau of Ordnance beginning in 1951 and continued through 1987, entire volumes were classified for decades), it isn't surprising that these fallacies persist. (These, along with the basic mechanical engineering texts, are the books the working military small arms designers reference when they are figuring something out concerning gun actions.) Few people have $1100 or so for those six books. Military small arms design engineering is a rather small field, so most of the good books have been out of print for 30-50 years or so. Instead, we get "experts" quoting from automotive repair manuals and using God-Only-Knows as their sources for these new and mechanically useless "definitions".
Defining short and long stroke systems based on the relative length of the cylinder versus it's width is appropriate for a Buick, but not a Kalashnikov. Trying to claim that an AR18 and an AKM are "exactly the same" and they are both "short stroke" is voodoo engineering not recognized by working ordnance designers.
For example, Eugene Stoner developed rifles with all three major gas systems -- direct impingement (AR15), short stroke gas (AR18), and long stroke gas (Stoner 63). He never had any problems differenciating between the three systems, and agreed that they all were valid (as opposed to saying the long stroke system was obsolete). The Russians have no problem differenciating between "long stroke" and "short stroke" based on whether or not the piston cycles the length of the minimum operating stroke. - - Geodkyt
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