.45 ACP in the context of "Semi-automatic pistol"

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 M2 machine gun or Browning .50-caliber machine gun (informally, "Ma Deuce") is a heavy machine gun that was designed near the end of World War I by John Browning. While similar to Browning's M1919 Browning machine gun, which was chambered for the .30-06 cartridge, the M2 uses Browning's larger and more powerful .50 BMG (12.7 mm) cartridge. The design has had many designations; the official U.S. military designation for the infantry type is Browning Machine Gun, Cal. .50, M2, HB, Flexible. It has been used against infantry, light armored vehicles, watercraft, light fortifications, and low-flying aircraft.

The gun has been used extensively as a vehicle weapon and for aircraft armament by the United States since the 1930s. It was heavily used during World War II, the Korean War, the Vietnam War, the Falklands War, the Soviet–Afghan War, the Gulf War, the Iraq War, and the War in Afghanistan. It is the primary heavy machine gun of NATO countries and has been used by many other countries as well. U.S. forces have used the M2 longer than any other firearm except the .45 ACP M1911 pistol, which was also designed by John Browning.

The Colt M1911 (also known as 1911, Colt 1911, Colt .45, or Colt Government in the case of Colt-produced models) is a single-action, recoil-operated, semi-automatic pistol chambered primarily for the .45 ACP cartridge.

A hollow-point bullet is a type of expanding bullet which expands on impact with a soft target, transferring more or all of the projectile's energy into the target over a shorter distance.

Hollow-point bullets are used for controlled penetration, where overpenetration could cause collateral damage (such as aboard an aircraft). In target shooting, they are used for greater accuracy due to the larger meplat. They are more accurate and predictable compared to pointed bullets which, despite having a higher ballistic coefficient (BC), are more sensitive to bullet harmonic characteristics and wind deflection.