5.56×45mm NATO in the context of "Caliber"

The M16 rifle (officially designated Rifle, Caliber 5.56 mm, M16) is a family of assault rifles adapted from the ArmaLite AR-15 rifle for the United States military. The original M16 rifle was a 5.56×45mm automatic rifle with a 20-round magazine.

In 1964, the XM16E1 entered US military service as the M16 and in the following year was deployed for jungle warfare operations during the Vietnam War. In 1969, the M16A1 replaced the M14 rifle to become the US military's standard service rifle. The M16A1 incorporated numerous modifications including a bolt-assist ("forward-assist"), chrome-plated bore, protective reinforcement around the magazine release, and revised flash hider.

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.

An assault rifle is a select fire rifle that uses an intermediate-rifle cartridge and a detachable magazine. Assault rifles were first put into mass production and accepted into widespread service during World War II. The first assault rifle to see major usage was the German StG 44, a development of the earlier Mkb 42. While immediately after World War II, NATO countries were equipped with battle rifles, the development of the M16 rifle during the Vietnam War prompted the adoption of assault rifles by the rest of NATO. By the end of the 20th century, assault rifles had become the standard weapon in most of the world's armies, replacing full-powered rifles and submachine guns in most roles. The two most successful modern assault rifles are the AK-47 and the M16 designs and their derivatives.

The Heckler & Koch G36 (German: Gewehr 36) is an assault rifle designed in the early 1990s by German weapons manufacturer Heckler & Koch. It is chambered in 5.56×45mm NATO, and replaced the heavier G3 battle rifle chambered in 7.62×51mm. The G36 was accepted into service with the Bundeswehr in 1997. Since then, it has been a popular export, and has seen active service in military and police units in several countries, including Germany, Spain, and the United Kingdom. The G36 is gas-operated and feeds from a 30-round detachable box magazine or 100-round C-Mag drum magazine.

In 2012, the G36 was found to suffer from significant accuracy issues due to thermal expansion of the barrel, prompting a search for a replacement. In 2017, the Bundeswehr launched the System Sturmgewehr Bundeswehr, a program designed to field a replacement for the G36. The weapons put forth were the Heckler & Koch HK416, Heckler & Koch HK433, and the Haenel MK 556. The G95A1 and G95KA1, both variants of the HK416, were selected in 2022.

The Heckler & Koch HK33 is a 5.56mm assault rifle developed in the 1960s by West German armament manufacturer Heckler & Koch GmbH (H&K), primarily for export.

Building on the success of their G3 design, the company developed a family of small arms (all using the G3 operating principle and basic design concept) consisting of four types of firearms: the first type, chambered in 7.62×51mm NATO; the second, using the Soviet 7.62×39mm M43 round; the third, chambered in .223 Remington and 5.56×45mm NATO; and the fourth type, chambered for the 9×19mm Parabellum pistol cartridge. Commercially the HK33 was a successful design but it did not sell as well as the G3.

The Heckler & Koch G41 is a German 5.56×45mm NATO assault rifle introduced in 1981 and produced in limited quantities by Heckler & Koch. It was designed to replace the 7.62×51mm NATO chambered Heckler & Koch G3 and the G3 based .223 Remington/5.56×45mm and later 5.56×45mm NATO chambered Heckler & Koch HK33 service rifles providing a more modern weapon compatible with then recently introduced NATO standards. It can use both the then new STANAG 4172 compliant 5.56×45mm NATO SS109, SS110, and SS111 ammunition and older .223 Remington/5.56×45mm M193 ammunition and was the last Heckler & Koch service rifle designed around the roller-delayed blowback mechanism.

Commercially the G41 was not as successful as the preceding Heckler & Koch service rifle designs with a similar operating mechanism. It was rejected by various military trial programmes and it never won a major military production contract. Assembly of the G41 has been discontinued by Heckler & Koch; however, production rights to the rifle were acquired by the Italian arms manufacturer Luigi Franchi.

The M14 rifle, officially the United States Rifle, Caliber 7.62 mm, M14, is an American battle rifle chambered for the 7.62×51mm NATO cartridge. It became the standard-issue rifle for the U.S. military in 1957, replacing the M1 Garand rifle in service with the U.S. Army by 1958 and the U.S. Marine Corps by 1965; deliveries of service rifles to the U.S. Army began in 1959. The M14 was used by the U.S. Army, Navy, and Marine Corps for Basic and Advanced Individual Training from the mid-1960s to the early 1970s.

The M14 was the last American battle rifle issued in quantity to U.S. military personnel. In 1967, it was officially replaced by the M16 assault rifle, a lighter weapon with a smaller 5.56×45mm intermediate cartridge. The M14 rifle remains in limited service across all branches of the U.S. military, with variants used as sniper and designated marksman rifles, accurized competition weapons, and ceremonial weapons by honor guards, color guards, drill teams, and ceremonial guards. Civilian semi-automatic variants are used for hunting, target shooting, and shooting competitions.