M27 Infantry Automatic Rifle
The M27 Infantry Automatic Rifle (IAR) is a 5.56mm, select-fire assault rifle, based on the HK416 by Heckler & Koch. It is used by the United States Marine Corps (USMC) and is intended to be issued to all infantry riflemen as well as automatic riflemen. The USMC initially planned to purchase 6,500 M27s to replace a portion of the M249 light machine guns employed by automatic riflemen within Infantry and Light Armored Reconnaissance Battalions. Approximately 8,000–10,000 M249s will remain in service with the Marine Corps to be used at the discretion of company commanders. In December 2017, the Marine Corps announced that it would equip every member of an infantry squad with the M27, supplanting the M4 carbine which would be retained at the platoon leadership positions and above.
M27 IAR with ACOG Squad Day Optic and AN/PEQ-16A weapon-mounted laser module with visible laser, infrared targeting laser, infrared illuminator, and white light
A US Marine armed with an M27 fitted with a Harris bipod and an ACOG Squad Day Optic covers his team in Afghanistan, March 2012.
A US Marine with a 2021-standard M27 fitted with a VCOG Squad Common Optic and a KAC suppressor.
A US Marine with an M27 fitted with an ACOG Rifle Combat Optic and an OKC-3S bayonet.
The 5.56×45mm NATO is a rimless bottlenecked intermediate cartridge family developed in the late 1970s in Belgium by FN Herstal. It consists of the SS109, L110, and SS111 cartridges. On 28 October 1980, under STANAG 4172, it was standardized as the second standard service rifle cartridge for NATO forces as well as many non-NATO countries. Though they are not entirely identical, the 5.56×45mm NATO cartridge family was derived from and is dimensionally similar to the .223 Remington cartridge designed by Remington Arms in the early 1960s.
5.56×45mm NATO with measurement, left to right: Bullet, case, and complete cartridge
The 7.62×51mm NATO and 5.56×45mm NATO cartridges compared to an AA battery
Service rifle cartridge cases: (left to right) 7.62×54mmR, 7.62×51mm NATO, 7.62×39mm, 5.56×45mm NATO, 5.45×39mm
5.56×45mm NATO cartridges in a STANAG magazine