Gas Systems
Most current gas systems employ some type of piston. The face of the piston is acted upon by gas from the combustion of the propellant from the barrel of the firearm. Early methods such as Browning's 'flapper' prototype, the Bang rifle, and Garand rifle used relatively low-pressure gas from at or near the muzzle. This, combined with more massive operating parts, reduced the strain on the mechanism. To simplify and lighten the firearm, gas from nearer the chamber needed to be used. This gas is of extremely high pressure and has sufficient force to destroy a firearm unless it is regulated somehow. Several methods are employed to regulate the energy. The M1 carbine incorporates a very short piston, or "tappet". This movement is closely restricted by a shoulder recess. Excess gas is then vented back into the bore. The M14 rifle and M60 GPMG use the White expansion and cutoff system to stop (cut off) gas from entering the cylinder once the piston has traveled a short distance. Most systems, however, vent excess gas into the atmosphere through slots, holes, or ports.
Read more about this topic: Gas-operated Reloading
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