Rate of Fire - Technical Limitations

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 Due to these disadvantages, water-cooled weapons have gradually been replaced by much lighter air-cooled weapons.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.