operation

All machine guns follow a cycle:

• Pulling (manually or electrically) the bolt assembly/bolt carrier rearward by way of the cocking lever to the point bolt carrier engages a sear and stays at rear position until trigger is activated making bolt carrier move forward
• Loading fresh round into chamber and locking bolt
• Firing round by way of a firing pin or striker (except for aircraft medium caliber using electric ignition primers) hitting the primer that ignites the powder when bolt reaches locked position.
• Unlocking and removing the spent case from the chamber and ejecting it out of the weapon as bolt is moving rearward
• Loading the next round into the firing chamber. Usually the recoil spring aka main spring tension pushes bolt back into battery and a cam strips the new round from a feeding device, belt or box.

Cycle is repeated as long as the trigger is activated by operator. Releasing the trigger resets the trigger mechanism by engaging a sear so the weapon stops firing with bolt carrier fully at the rear.

The operation is basically the same for all semi automatic or automatic weapons, regardless of the means of activating these mechanisms. Some examples:

• Machine pistols and submachine guns (like the World War II "grease gun", MAC-10 or the Uzi) are usually blowback operated.

direct impingement

gas piston

• Most assault rifles and squad automatic weapons are gas operated. Some weapons, such as the AR-15/M16, do not have a piston, instead using a system of direct impingement in which the gases operate the bolt carrier by acting directly on it. Others, like the original SA80 patterns, have a bolt carrier that is unlocked and operated by a piston actuated by gases.

• A recoil actuated machine gun uses the recoil to first unlock and then operate the action. Heavy machine guns, such as the M2 .50 and Browning .50, are of this type. A cam, lever or actuator demultiplicates the energy of the recoil to operate the bolt.

• An externally actuated machine gun uses an external power source, such as an electric motor or even a hand crank to move its mechanism through the firing sequence. Most modern weapons of this type are called Gatling guns in reference to their driving mechanism. Gatling guns and revolver cannon have several barrels or chambers on a rotating carousel and a system of cams that load, cock, and fire each mechanism progressively as it rotates through the sequence. The continuous nature of the rotary action allows for an incredibly high cyclic rate of fire, often several thousand rounds per minute. Not all rotary guns use multiple barrels or chambers, though. Rotary guns are less prone to jamming than a gun operated by gas or recoil, as the external power source will eject misfired rounds with no further trouble. This is not possible if the force needed to eject the round comes from the round itself. Rotary guns are generally used with large shells, 20 mm in diameter or more They offer benefits of reliability and firepower, though the weight and size of the power source and driving mechanism makes them impractical for use outside of a vehicle or aircraft mount.

Heavy machine guns were often water cooled but air cooled machine guns have interchangeable barrels, which must be changed periodically to avoid overheating. The higher the rate of fire, the more often barrels must be changed and allowed to cool. To minimize this, most air-cooled guns are fired only in short bursts or at a reduced rate of fire.

In weapons where the round seats and fires at the same time, mechanical timing is essential for operator safety, to prevent the round from firing before it is seated properly. Machine guns are controlled by one or more mechanical sears. When a sear is in place, it effectively stops the bolt at some point in its range of motion. Some sears stop the bolt when it is locked to the rear. Other sears stop the firing pin from going forward after the round is locked into the chamber.

Almost all weapons have a "safety" sear, which simply keeps the trigger from engaging.