There are some similarities between recoil- 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, inmost 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.

Thus the weight of the breech bolt is an important factor in the design and operation of a blowback-operated weapon. When used with low-powered ammunition, it is a suitable arrangement. A military rifle, however, using the standard .30-cal. cartridge and the blowback action, would require a 27-pound breechblock.

Besides the submachine gun, many types of so-called pocket automatic pistols and .22-cal. automatic rifles use blowback operations.

There are variants in the methods used for each of these types to operate the mechanism for blowback. These are the straight blowback, retarded blowback, and accelerated blowback methods.

STRAIGHT BLOWBACK METHOD.-

Straight blowback is the most elementary and simple. It uses recoil energy from the firing of a round of ammunition to operate the mechanism of the weapon and extract the fired case, eject it against spring tension, and return the mechanism to firing position again. This, in turn, picks up an unfired round from a magazine and chambers it. Straight blowback is used in weapons that fire ammunition of fairly low power, such as pistol ammunition and .22-cal. rimfire rifle cartridges. The bolt slide or breechblock is fairly heavy in these weapons when compared to the weight of the bullet and power of the cartridge. Therefore, the mechanism will stay closed (but not locked) momentarily until the bullet gets free of the barrel and pressure is subdued to allow extraction. All submachine guns and semiautomatic .22-cal. rimfire pistols use straight blowback for their operation.

RETARDED BLOWBACK METHOD.- An example of retarded blowback is found in the mechanism of the original Thompson submachine gun. This is based on the principle of operation that the recoil force exerted on the mechanism must overcome some form of mechanical disadvantage, momentarily holding the breechblock closed until the bullet had cleared the muzzle of the weapon. However, this was later found unnecessary if the bolt or breechbhck was of sufficient weight. The Thompson Ml-Al submachine gun (formerly used by the Navy) uses straight blowback as have all submachine guns designed since that time.

ACCELERATED BLOWBACK METHOD.-

An example of accelerated blowback is found in the .22-cal. rimfire Colt Ace semiautomatic pistol. In this pistol, the Williams floating chamber, apart of the barrel on firing a round of ammunition, moves with accelerated force against the mechanism (in this case, the fairly heavy slide and its components), providing sufficient energy to operate the component parts of a .45-cal. pistol with .22-cal. rirnfire ammunition.

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 as the cyclic rate of fire and the sustained rate of fire.

MAXIMUM RANGE.- Maximum range is the greatest distance that the projectile will travel.

MAXIMUM EFFECTIVE RANGE.- Maximum effective range is the greatest distance at which a weapon may be expected to fire accurately to inflict damage or casualties.

CYCLIC RATE OFFIRE.- The cyclic rate of fire is the maximum rate at which a weapon will fire in automatic operation, stated in rounds per minute (rpm).

SUSTAINED RATE OF FIRE.- The sustained rate of fire of a weapon is normally stated in a chart. The chart correlates the average number of rounds fired per minute with the number of minutes this rate can be sustained without damage to the weapon.