Rate of fire

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Rate of fire is the frequency at which a specific weapon can fire or launch its projectiles. It is usually measured in rounds per minute (RPM or round/min), or per second (RPS or round/s).

Note that both generation of heat (leading to weapon failure) and exhaustion of the weapon's ammunition mean that most automatic weapons are unlikely ever to sustain their cyclic rate of fire for a full minute; thus, it is technically incorrect and potentially misleading to describe RPM as "the number of rounds a weapon can fire in one minute."

For manually-operated weapons such as bolt-action rifles or artillery pieces, the rate of fire is governed primarily by the training of the operator or crew, within some mechanical limitations. Rate of fire may also be affected by ergonomic factors. For rifles, ease-of-use features such as the design of the bolt or magazine release can affect the rate of fire. For artillery pieces, a gun on a towed mount can usually achieve a higher rate of fire than the same weapon mounted within the cramped confines of a tank or self-propelled gun. This is because the crew operating in the open can move more freely and can stack ammunition where it is most convenient. Inside a vehicle, ammunition storage may not be optimized for fast handling due to other design constraints, and crew movement may be constricted.

For automatic weapons such as machine guns, the rate of fire is primarily a mechanical property.

Over time, weapons have attained higher rates of fire. A small infantry unit armed with modern assault rifles and machine guns can generate more firepower than much larger units equipped with older weapons. Over the past century, this increased firepower is due almost entirely to the higher rate of fire of modern weapons.

A good past example of growth in rate of fire would be the enormous advantage of the Maxim machine gun, which provided accurate and steady fire. This was due to technological advances in the field of material cooling.

Measurement

There are three standard measurements of rate of fire for automatic weapons:

• Cyclic rate: This is the mechanical rate of fire, or how fast the weapon "cycles" (loads, locks, fires, unlocks, ejects). Measurement of the cyclic rate assumes that the weapon is being operated as fast as possible and does not consider operator tasks (magazine changes, aiming, etc). When the trigger is squeezed, the rate at which rounds are fired is the cyclic rate. Typical assault rifles have a cyclic rate of 500–800 RPM. Typical infantry machine guns have rates varying from 600 RPM to 1,200 RPM. M134 Miniguns mounted on helicopters can achieve rates of fire as high as 50 rounds per second (3,000 RPM).

• Effective rate: In contrast to the cyclic rate, the effective rate is the actual rate at which the weapon would typically be fired in combat. Effective rate measurement starts with the cyclic rate, but considers other factors as well, such as time spent reloading, aiming, changing barrels if necessary, and allowing for some cooling. Machine guns are typically fired in short bursts rather than in long continuous streams of fire, although there are times when they must be fired in very long bursts. The effective rate is always lower than the cyclic rate.

• Sustained rate: The rate at which the weapon could reasonably be fired indefinitely without failing. Knowing the sustained rate of fire is useful to know for logistics and supply purposes. It is most often used when continuous fire is desired for long periods of time as with artillery.

Technical limitations

The major limitation in higher rates of fire arises due to the problem of heat. Even a manually-operated rifle generates heat as rounds are fired. A machine gun builds up heat so rapidly that steps must be taken to prevent overheating. Solutions include making barrels heavier so that they heat up more slowly, making barrels rapidly replaceable by the crews, or using water jackets around the barrel to cool the weapon. A modern machine gun team will carry at least one spare barrel for their weapon, which can be swapped out within a few seconds by a trained crew. Problems with overheating can range from ammunition firing unintentionally (cook-off), or, what is much worse in combat, failure to fire.

Water-cooled weapons can achieve very high effective rates of fire (approaching their cyclic rate) but are very heavy and vulnerable to damage. A well-known example is the M2 Browning heavy machine gun, produced in both air-cooled version and water-cooled versions: the former weighed 38 kg (84 lb), while the latter weighed 66 kg (121 lb) including coolant.[1] Due to these disadvantages, water-cooled weapons have gradually been replaced by much lighter air-cooled weapons. For weapons mounted on aircraft, no cooling device is necessary due to the outside air cooling the weapon as the aircraft is moving. Consequently, aircraft-mounted machine guns, autocannon or miniguns can sustain fire far longer than ground-based counterparts, firing close to their cyclic rate of fire.

Another factor influencing rate of fire is the supply of ammunition. At 100 RPS, a one-second burst from the M134 would use approximately 2.5 kg (5 lb) of 7.62 mm ammunition; this alone would make it an impractical weapon for infantry who have to carry a reasonable supply of ammunition with them. For this and other reasons, weapons with such high rates of fire are typically only found on vehicles or fixed emplacements.