Most of the self-loading rifles designed during and after WWII have been gas-operated. These contrast with blowback and recoil systems in that operating energy comes from tapping expanding high-pressure gases at the barrel instead of from recoil. Examples include the U.S. M1 Garrand, M1A, and M1 Carbine, Simonov rifle (SKS), Automat Kalashnikov (AK), Swedish Ljungman, the late Eugene Stoner's AR10 and AR15 (M16), FN FAL, M60 and M249 Minimi machine guns. (Perhaps the most notable exceptions are the Heckler & Koch delayed blowback designs described in that section. The HK system arguably has had more impact in the submachine gun realm than among rifles.)
Gas-operated designs vary in how the gas is tapped, and how gas energy is transferred to the bolt carrier. All listed above except the FAL use rotary locking bolts which follow helical cam tracks in their carriers or receivers. In the Garrand, SKS and AK families, and most modern light to medium machine guns, gas impinges directly on operating rods fixed to the bolt carrier and located below or above the barrel. In the Ljungman and Stoner designs a narrow steel tube carries the pressurized gas back to small operating cylinders on the bolt and carrier. In the M1 carbine and more recent designs such as Eugene Stoner's Amarlite AR-18, Australia's Leader, Singapore's Sterling-designed SAR rifle family, Korea's Daewoo rifle, and Heckler & Koch's 1998 G36, a small, low-mass tappet is the only moving part in contact with the gas. (The latter designs are essentially derived from Stoner's work.) The tappet accelerates rapidly due to its low mass and imparts its momentum by striking an operating rod connected to or striking the bolt carrier.
Because gas residue can potentially jam the system and be difficult to dislodge, most gas operating systems are claimed to posses self-cleaning or non-user-maintenance parts. For example the gas tube on the AR15 is not a user maintenance item, though the cylinder and other bolt parts it operates on are meant to be cleaned. The gas tube is not replaced under regular maintenance but can be if the armorer deems necessary. After correcting propellant materials incompatible with the jungles of Vietnam, the AR15 gas system has worked well. The gas piston on the AK has sharp forward edges which in theory scrape burnt propellant buildup off the walls of the gas cylinder and vent it out the relief ports.
Pressure at the gas port rises quickly when the bullet passes the port, and it begins to drop after pressures in the barrel and gas cylinder equalize. The barrel's gas port must be far enough from the breech to allow pressures to fall to safe levels before the breech begins to open. The main way to influence this timing is to adjust the distance from the breech to the gas port. For rifles the distance is usually at least 10 inches or 25 centimeters. Breech opening can be delayed further through mechanical disadvantage, free travel in unlocking mechanisms, recoil dampers, bolt mass, and so on.
Perhaps the cleanest solution to gas operation (pun?) is the tappet which localizes gas and propellant residue in the tappet/cylinder area and away from the rest of the system. However, long operating rods used with tappets or direct impingement increase and spread out the moving mass, which tends to shift the point of balance when operated. The tappet and cylinder are sometimes sealed as a unit and are often claimed to be self-cleaning.
In contrast, the AR15 system with it's relatively light and quick-moving bolt and springs is very pleasant to fire, with perceived recoil not much more than a .22 rimfire rifle. The low felt recoil also derives from the light-recoiling 5.56mm NATO (.223 Remington) round the AR15 is traditionally chambered for. (The AR15 has been adapted to 9mm, 7.62mm, and probably other calibers by Colt's Manufacturing, Stoner/Knights, ArmaLite (Eagle Arms).) At the time of its original design, the plastic furniture and aluminum forgings of the AR15 made for a radically lightweight military rifle. Stoner's ground breaking design remains successful more than 40 years after initial development.
While more accurate due to better manufacture and a tighter-tolerance design, the AR15 cannot match the simplicity of the AK system. The AK system has been copied or licensed and is manufactured in greater volume than any other military rifle design. Licensed models are produced in many communist or former Soviet-aligned nations throughout the world. Some Western AK derivatives include Finnish Models 60 and 62/76 made by Valmet, the Valmet-derived Israeli Galil, and the Galil-derived South African R4.
The AK uses the popular and successful design features described above of rotating bolt and fixed operating rod attached to the bolt carrier. The bolt has a single follower which engages a cam track in the receiver and a single locking lug. Most AK receivers are stamped and welded sheet metal with blued or painted finishes. The top cover is traditionally sheet metal. Furniture is usually wood and sometimes plastic. Frame parts at the front and rear of the receiver are cast or machined and riveted to the receiver.
The AK bolt assembly is driven by a helical spring surrounding a rod attached to a removable carrier behind the bolt. The rear of the spring/rod carrier forms the latch which retains the top cover. The spring carrier also keeps the bolt carrier on guide rails in the receiver. Disassembly is accomplished by pressing the latch, removing the cover, withdrawing the spring/rod carrier and pulling back then lifting up the bolt carrier from the stamped sheet steel guide rails. That this is accomplished with two major parts groups and can be performed in less time than it takes to describe is a testament to the elegant simplicity of the design. The overall simplicity of the system also lends outstanding reliability in dusty or humid environments found in desert or jungle.
As with most engineering issues there is no singularly perfect solution. All have tradeoffs which must be weighed against each other and evaluated in the context of the system and expected operating environments. One thing is clear however: simpler is better.
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