MK18 (ID 8.41)

The Mk 18 is a short-barreled variant of the M4 carbine. Chambered in 5.56x45mm NATO and features a 262mm (10.3in) barrel compared to the M4’s (14.5in) barrel. It is the weapon of choice for CQB when the maximum engagement distance is expected to be less than 300m. The Mk 18 was originally known as the Close Quarters Battle Receiver (CQBR). It was developed by the US Naval Surface Warfare Center, Crane Division as a replacement upper receiver for M4 carbines issued to Navy SEALs. The M4 splits in half into two receivers, the upper and the lower. The upper contains the barrel and bolt carrier group, while the lower contains the stock and fire control group. The idea behind the CQBR program was to allow the user to quickly swap upper receivers based on mission parameters. However, the CQBR program evolved from a simple upper receiver replacement into a complete firearm package: the Mark 18.