AUTOMATIC AND SEMIAUTOMATIC FIRING SYSTEMS
LEARNING OBJECTIVE Discuss the operation and maintenance of Navy small arms.
A semiautomatic weapon unlocks, extracts, ejects, cocks, and reloads automatically. However, the trigger must be pulled each time to fire a round. By this definition, the .45-cal. M1911A1 pistol is semiautomatic, though often called automatic. A fully automatic weapon keeps on firing as long as the trigger is kept pulled.
Two examples of weapons that can be fired both automatically and semiautomatically are the 7.62-mm M14 rifle and the 5.56-mm M16 rifle.
SMALL-ARMS OPERATING PRINCIPLES
Automatic and semiautomatic weapons are classified on the basis of how they obtain the energy required for operation. Fundamentally, small arms obtain the energy from the forces that accompany the explosion created when around of ammunition is fired. The use of these forces does not reduce the effectiveness of the weapon but uses otherwise wasted energy.
There are three basic types of operation for semiautomatic and automatic small arms-gas operation, recoil operation, and blowback operation. Figure 3-2 shows the three methods.
In gas-operated weapons, a portion of the expanding powder gases behind the bullet is tapped off into a gas cylinder located beneath the barrel. (The hole connecting the barrel and cylinder is near the muzzle end.) As the bullet passes this hole, gases push this piston rearward. The piston is connected by a rod to an operating mechanism of the weapon, such as the bolt. The piston carries the bolt aft with it, unlocking, extracting, ejecting, and cocking the weapon.
Three basic types of gas systems are used in semiautomatic and automatic small arms. They are the gas impingement, gas tappet, and gas expansion systems.
GAS IMPINGEMENT SYSTEM.- The impingement system has a negligible volume of gas at the cylinder with expansion dependent on piston motion. As the piston moves, gas continues pouring through the port until the bullet exits the muzzle with a subsequent drop in pressure in the bore. An example of such a mechanism is found in the Ml Garand rifle, which was the standard service rifle in World War II and Korea.
Figure 3-2.-Types of operating systems.
GAS TAPPET SYSTEM.- The gas tappet system is an impingement system with a short piston travel. It is often referred to as a gas short stroke system. An example of such a mechanism is found in the Ml and M1A1 .30-cal. carbine. In some tappet mechanisms, the piston only taps the lock mechanism open and exerts no force to recoiling components.
GAS EXPANSION SYSTEM.- The gas expansion system, in contrast to the impingement system, has an appreciable initial volume of gas in its expansion chamber. This requires more time to pressurize the chamber and also more time to exhaust the gas by selection of port size and location as the required pressurized gas can be drained from the bore.
There is also a cutoff expansion that is similar to the direct expansion system, except for a valve that closes the port after the piston moves. As the pressure builds up to a specific value, the piston moves, closing the port and leaving the gas to expand, providing the force effort needed to operate the moving components. The 7.62-mm M14 rifle uses this type of operation.
As a round is fired, high pressures develop behind the bullet and force it down the barrel. The force behind the bullet is also directed rearward against the breech. If the barrel and bolt are secured to one another, the entire force of recoil is felt on the shooter's shoulder. But, by designing the barrel and breech assembly so that they can slide in the frame or receiver, the energy of the
rear moving assembly can be used to compress springs, move levers, and soon, necessary to complete the cycle of operation.
Generally, in recoil-operated weapons, the barrel and the bolt move rearward together for a short distance. Then the barrel is stopped and the bolt (now unlocked) continues to the rear against spring pressure until the empty case is ejected. The force of recoil is also used to cock the weapon and compress the spring, returning the bolt to its firing position and cambering a new round in the process.
There are two basic methods of recoil operation for semiautomatic and automatic small arms. They are the long-recoil (Browning) and short-recoil (Maxim) methods.
LONG-RECOIL METHOD.- The dynamics of long-recoil-operated weapons are similar to straight blowback operation, except that the barrel, breechblock, and component parts recoil together for the complete recoil cycle. This recoil distance must be greater than the length of the complete round. At the end of the recoil stroke, the bolt is held while the barrel counterrecoils alone. One particular note of importance on the long-recoil type of operation is that ejection takes place on counterrecoil instead of recoil. An example of a long-recoil weapon is the Browning designed, Remington model 11 shotgun, used by the Navy before and during World War II.
SHORT-RECOIL METHOD.- The dynamics of short-recoil-operated weapons approach those of the retarded blowback types more nearly than long-recoil. The bolt latch is not released until the propellant gases become ineffective to eliminate all blowback
tendencies. After unlatching (unlocking), the bolt continues recoiling and in some mechanisms is accelerated by mechanical or gas systems. The barrel is arrested by spring, buffer, stop, or a combination of these and is caused to return to battery by these or the counterrecoiling components. Examples of short-recoil-operated weapons are the .45-cal. pistol and the Browning machine gun.