Machine gun

A machine gun is a fully automatic, rifled firearm. It is specifically designed for sustained direct fire. Unlike semi-automatic guns, which fire one round per trigger pull, a machine gun continues firing as long as the trigger is held. These weapons are primarily used for infantry support. Because they produce significant recoil, they are often used with a bipod or a tripod. They may also be placed on a fixed mount or a heavy weapons platform to maintain stability.

Most modern machine guns are of the locked breech type. They follow a specific mechanical cycle to function. First, the bolt carrier moves forward to load a fresh round into the chamber. Once the bolt is locked, a firing pin or striker hits the primer. This ignites the propellant powder inside the cartridge. After the shot, the weapon must unlock and remove the spent case. This is done by moving the bolt rearward. A recoil spring then pushes the bolt back into battery. During this movement, a cam strips a new round from a belt or magazine. This entire cycle repeats rapidly as long as the operator holds the trigger.

To keep this cycle moving, machine guns use different energy sources. Many utilize gas-operated reloading. This process taps off propellant gas from the fired cartridge. The mechanical pressure of that gas unlocks the bolt and cycles the action. The first patent for a gas-operated rifle was held by the French brothers Claire in 1892. Another common method is recoil actuation. This type uses the energy from the gun's recoil to operate the mechanism. Famous examples include the M2 Browning and the MG42.

Machine guns are categorized into several distinct types. Light machine guns provide mobile support for a squad. They are typically air-cooled and use a bipod with a box magazine or drum. Medium machine guns use full-sized rifle rounds and are usually mounted on a tripod. General-purpose machine guns are versatile weapons. They can act as light machine guns with a bipod and drum, or as medium machine guns with a tripod and belt feed. Heavy machine guns use larger calibers. Today, the term refers to automatic weapons with a caliber of at least .50 but less than 20 mm.

Managing heat is a critical part of machine gun design. Firing many rounds creates extreme temperatures. If a cartridge gets too hot, it might detonate without a trigger pull. This dangerous event is called a cook-off. To prevent this, engineers use various cooling systems. Early models often used water-cooled jackets. While effective, these added great weight and were vulnerable to enemy fire. Modern designs are mostly air-cooled. They may use heavy barrels to soak up heat, passive cooling fins, or removable barrels. A crew can simply swap a hot barrel for a cool one to keep firing.

Some specialized designs use external power to increase performance. These are often called Gatling guns, named after the original inventor. These weapons use multiple barrels on a rotating carousel. An external motor or hand crank moves the mechanism through the firing sequence. This rotary action allows for a very high cyclic rate of fire. Some can fire several thousand rounds per minute. Because they are bulky and heavy, they are usually mounted on aircraft or vehicles. Large-caliber rotary guns, often called rotary cannons, typically use rounds that are 20 mm or larger.

Machine guns play a significant role in various military operations. They are used against infantry, small boats, and lightly armored vehicles. They can also provide suppressive fire to control an area of land. Some heavy machine guns are incredibly accurate at long distances. During the Vietnam War, Carlos Hathcock set a long-distance record of 2,286 yards using a .50 caliber machine gun with a telescopic sight. This capability led to the development of .50 caliber anti-materiel sniper rifles. These weapons connect the role of the machine gun to broader fields of long-range precision engagement.