Question: How fast is the fastest firing machine gun?

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).
Several different measurements are used. The fastest and most commonly quoted rate is the cyclical rate of fire. However, heat (possibly leading to weapon failure) and exhaustion of ammunition mean most automatic weapons are unlikely to sustain their cyclic rate of fire for a full minute. That is why lower rates apply in practice and it is incorrect to describe the cyclic rate of fire as "the number of rounds a weapon can fire in one minute."
Overview
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. Artillery rates of fire were increased in the late 19th century by innovations including breech-loading and quick-firing guns.
For automatic weapons such as machine guns, the rate of fire is primarily a mechanical property. A high cyclic firing rate is advantageous for use against targets that are exposed to a machine gun for a limited time span, like aircraft or targets that minimize their exposure time by quickly moving from cover to cover. For targets that can be fired on by a machine gun for longer periods than just a few seconds the cyclic firing rate becomes less important.
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 selective fire feature that 'downgrades' them to semi-automatic mode by changing a switch.
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 20th century, this increased firepower was due almost entirely to the higher rate of fire of modern weapons.
A good example of growth in rate of fire is the Maxim machine gun that was developed in 1884 and used until World War I ended in 1918.  Its performance was improved during that time mainly by advances in the field of cooling.
Measurement
There are diverse measurements of rate of fire. The speed of the fire will vary depending on the type of automatic weapon.
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 pulled, the rate at which rounds are fired is the cyclic rate. Typical cyclic rates of fire are 600–900 RPM for assault rifles, 1,000-1,100 RPM in some cases, 900-1,200 RPM for submachine guns and machine pistols, and 600-1,500 RPM for machine guns. M134 Miniguns mounted on attack helicopters and other combat vehicles can achieve rates of fire of over 100 rounds per second (6,000 RPM). Cyclic rate of fire is the only rate that can be determined precisely.
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 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 12-15rpm; 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 great amount of ammunition (more than the troops are likely to carry on a patrol), the heat generated requires barrel change times to be reduced, and with the one spare barrel usually issued, prolonged rapid fire will result in shortened weapon/barrel life.
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 or explosion of the weapon.
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, a heavy machine gun designed by John Browning and used by US forces during WWI. It became the basis of the much more common Browning M1919 machine gun, used by US forces throughout World War II, as well as the Browning M2 .50 caliber heavy machine gun, which is still in service, as well as many adaptions, such as the Japanese Ho-103 aircraft machine gun during World War II.[1] Another legendarily reliable heavy machine gun is the British Vickers machine gun, based on the Maxim machine gun design, which saw service both on the air and ground during World War I and World War II. Due to their 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 Gatling-type guns can sustain fire far longer than ground-based counterparts, firing close to their cyclic rate of fire. However, due to the weight of the ammunition, many aircraft cannons only carry enough ammunition for a few seconds' worth of firing (though some can do a lot more, such as the GAU-8 mounted on the A-10 Thunderbolt, which can fire for 17 seconds with about 1,150 rounds of ammunition).
Another factor influencing rate of fire is the supply of ammunition. At 50 RPS (3,000 RPM), a five-second burst from the M134 would use approximately 6.3 kilograms (14lb) of 7.62mm 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.
See also
Multiple barrel firearm