Rate of fire

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 rounds per second (RPS or round/s).

Overview
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.

For a third hybrid class of weapons, common in handguns and rifles, known as a Semi-automatic firearm, the rate of fire is primarily governed by the ability of the operator to actively pull the trigger. No other factors significantly contribute to the rate of fire. Generally, a semi-automatic firearm automatically chambers a round using blowback energy, but does not fire the new round until the trigger is released to a reset point and actively pulled again. Semi-automatics' rate of fire is significantly different from and should not be confused with full-automatics' rate of fire. Many full-automatic small arms have a select-fire feature that allows them to be 'downgraded' to semi-automatic mode by changing a switch, but the reverse is not true, semi-automatics do not have a full automatic mode without gunsmith-level modifications.

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 cyclic rates of fire are 460–900 RPM for assault rifles, 1,000-1,100 RPM in some cases, 900-1,200 RPM for submachine guns and machine pistols, and 600-1,200 RPM for machine guns. M134 Miniguns mounted on helicopters can achieve rates of fire of over 100 rounds per second (6,000 RPM).

Sustained or Effective rate
This is the rate at which the weapon could reasonably be fired indefinitely without failing. In contrast to the cyclic rate, the sustained rate is the actual rate at which the weapon would typically be fired in combat. Sustained rate considers several factors, time spent reloading, aiming, changing barrels if necessary, and allowing for some cooling. Knowing the sustained rate of fire is useful to know for logistics and supply purposes. 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 (see rapid rate below). Sustained rate also applies to box magazine fed assault rifles and semi-automatic rifles. In these weapons it refers to the rate at which the typical rifleman can effectively engage targets in a combat situation. The rate is usually 12-15 rpm, except for barrel changes it considers most of the same factors as for the belt fed MGs.

Rapid rate
Rapid rate is a rate of fire between Cyclic and Sustained. It is usually much faster, although less accurate, than the sustained rate and is only used in emergency/final defensive line situations. The Rapid rate is not sustainable for long periods because it eats up a significant amount of ammunition (more than the gunner or assistant gunner are likely to carry on a patrol), the heat generated requires barrel change times to be reduced, and because machine guns are only issued with one spare barrel prolonged rapid fire will result in shortened weapon/barrel life. The Neal submachine gun came with 5 barrels to overcome this problem.

Semi-Automatic rate
The semi-automatic rate is the assault rifle/semi-auto only version on rapid fire. It is the maximum rate that a weapon can fire with any degree of accuracy in semi-auto mode, usually 45-60 rpm. However a semi-automatic firearm can mechanically function at (about) the same rate of fire as its fully automatic (or select-fire) counterpart, this is assuming the trigger could be pulled fast enough to facilitate such a rate of fire.

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 M1917 Browning machine gun heavy machine gun, produced in both .30-06 Springfield version and .50 BMG versions: the former weighed 38 kg (84 lb), while the latter weighed 66 kg (121 lb) including coolant. 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 50 RPS, a five-second burst from the M134 would use approximately 2.5 kg (5.5 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.