AUTOMATIC AND SEMIAUTOMATIC FIRING SYSTEMS

LEARNING OBJECTIVES: Explain the difference between automatic and semiautomatic firing systems. Identify and explain the three basic types of operation for small arms weapons. Distinguish between range and rates of fire.

A semiautomatic weapon unlocks, extracts, ejects, cocks, and reloads automatically, However, the trigger must be pulled each time to fire around. 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 find 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 energy from the forces that accompany the explosion created when a round 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 weapons: gas operated, recoil operated, and blowback operated. Figure 8-2 shows the three methods.

Gas-Operated Weapons

In gas-operated weapons, a portion of the expanding powder gases behind the bullet are 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 a 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

Figure 8-2.-Types of operation for semiautomatic andautomatic small arms weapons.

the bolt, extracting and ejecting the ammunition, and cocking the weapon.

Recoil-Operated Weapons

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 design, the barrel and breech assembly slide in the frame so that the rear moving assembly maybe used to compress springs or move levers, which completes 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.

Blowback-Operated

There are similarities between recoil-operated and blowback-operated weapons. But there are several major differences. In recoil operation, the bolt and barrel are locked together until the bullet has left the barrel, and most of the recoil thrust is spent. The combined thrust of the recoiling barrel, bolt, and some other parts is used to operate the weapon. In blowback (inertia) operation, however, the bolt is not locked to the barrel and in most cases the barrel does not recoil, The bolt is held closed by spring pressure and the mass of the breechblock. The initial blow of the exploding cartridge starts the bolt moving rearward, but the weight of the bolt is such that it does not allow the chamber to be entirely opened until the round has left the bore. Action by a recoil spring returns the bolt to the closed position, cambering a new round.

Range and Rate of Fire

Some other important terms that apply to small arms describe their range and rate of fire. The range of a weapon is stated in terms of maximum range and maximum effective range. The rate of fire of an automatic weapon is stated in terms of cyclic rate of fire or sustained rate of fire.

MAXIMUM RANGE- The greatest distance the projectile will travel.

MAXIMUM EFFECTIVE RANGE- The greatest distance at which a weapon may be expected to fire accurately to inflict damage or casualties.

CYCLIC RATE OF FIRE- The maximum rate at which a weapon will fire in automatic operation, stated in rounds per minute (RPM).

Figure 8-3.-.45-cal. M1911A1 semiautomatic pistol: A. Slide closed; B. Slide open.