The M4 carbine is a scaled-down version of the M16 rifle. It fires the same 5.56x45mm ammunition from the same magazines.
The M4A1 is an Assault Carbine used by U.S. troops as well as the Ghosts. It is basically a M4 that has a fully-automatic firing mode in place of a 3-Round burst firing mode. Specially for the SOCOM M4A1s, US Naval Surface Warfare Center developed a SOPMOD M4 kit, that consisted of the M4A1 carbine equipped with Rail Interface System (RIS) instead of the standard handguards. The kit also includes a variety of the add-on goodies, such as various sights (ACOG 4X telescopic, ACOG Reflex red-dot, detachable back-up open sights), laser pointers (visible and infra-red), detachable sound suppressor (silencer), modified M203 40mm grenade launcher (with shortened barrel and improved sights). The kit also included a detachable front grip and tactical light.