It's like a normal M4A1 except with an 11.5" barrel (old style stock, M4 handguards).
It's since been superseded by the Mk18 mod 0 and mod 1 (CQB-R)
edit: based on the description it sounds more like it was an NCO/Officer weapon (i.e., people not usually assigned a rifle). MFR operators usually carry a 1911 (MEU SOC 45).
Last edited by Drake; June 11th, 2012 at 19:40..