.50 BMG in the context of "M2 Browning"

Ammunition, also known as ammo, is the material fired, scattered, dropped, or detonated from any weapon or weapon system. The term Ammunition includes both expendable weapons (e.g., bombs, missiles, grenades, land mines), and the component parts of other weapons that create the effect on a target (e.g., bullets and warheads).

The purpose of ammunition is to project a force against a selected target to have an effect (usually, but not always, lethal). An example of ammunition is the firearm cartridge, which includes all components required to deliver the weapon effect in a single package. Until the 20th century, black powder was the most common propellant used but has now been replaced in nearly all cases by modern compounds.

In guns, particularly firearms, but not artillery, where a different definition may apply, caliber (or calibre; sometimes abbreviated as "cal") is the specified nominal internal diameter of the gun barrel bore – regardless of how or where the bore is measured and whether the finished bore matches that specification. It is measured in inches or in millimeters.[1] Since metric and US customary units do not convert evenly at this scale, metric conversions of caliber measured in decimal inches are typically approximations of the precise specifications in non-metric units, and vice versa.

In a rifled barrel, the distance is measured between opposing lands or between opposing grooves; groove measurements are common in cartridge designations originating in the United States, while land measurements are more common elsewhere in the world. Measurements "across the grooves" are used for maximum precision because rifling and the specific caliber so measured is the result of final machining process which cuts grooves into the rough bore, leaving the "lands" behind.

The North American Aviation P-51 Mustang is an American long-range, single-seat fighter and fighter-bomber used during World War II and the Korean War, among other conflicts. The Mustang was designed in 1940 by a team headed by James H. Kindelberger of North American Aviation (NAA) in response to a requirement of the British Purchasing Commission. The commission approached NAA to build Curtiss P-40 fighters under license for the Royal Air Force (RAF). Rather than build an old design from another company, NAA proposed the design and production of a more modern fighter. The prototype NA-73X airframe was completed on 9 September 1940, 102 days after contract signing, achieving its first flight on 26 October.

The Mustang was designed to use the Allison V-1710 engine without an export-sensitive turbosupercharger or a multi-stage supercharger, resulting in limited high-altitude performance. The aircraft was first flown operationally by the RAF as a tactical-reconnaissance aircraft and fighter-bomber (Mustang Mk I). In mid 1942, a development project known as the Rolls-Royce Mustang X, replaced the Allison engine with a Rolls-Royce Merlin 65 two-stage inter-cooled supercharged engine. During testing at Rolls-Royce's airfield at Hucknall in England, it was clear the engine dramatically improved the aircraft's performance at altitudes above 15,000 ft (4,600 m) without sacrificing range. Following receipt of the test results and after further flights by USAAF pilots, the results were so positive that North American began work on converting several aircraft developing into the P-51B/C (Mustang Mk III) model, which became the first long-range fighter to be able to compete with the Luftwaffe's fighters. The definitive version, the P-51D, was powered by the Packard V-1650-7, a license-built version of the two-speed, two-stage-supercharged Merlin 66, and was armed with six .50 caliber (12.7 mm) AN/M2 Browning machine guns.

A machine gun (MG) is a fully automatic and rifled firearm designed for sustained direct fire. Automatic firearms of 20 mm (0.79 in) caliber or more are classified as autocannons rather than machine guns.

As a class of military kinetic projectile weapons, machine guns are designed to be mainly used as infantry support weapons and generally used when attached to a bipod or tripod, a fixed mount or a heavy weapons platform for stability against recoil. Many machine guns also use belt feeding and open bolt operation, features not normally found on other infantry firearms.