Yay for new Black Lagoon fan art!

Nice pic, strange gun, I usually recognize most, but this one, I dont recognize at all.

Standard issue M4 carbine.

Tagged as M4 carbine, although I'm only taking Crotalus' word for it and can't yet distinguish between M4 and the many variants of M16. :-P

M4 quick recognition guide: 14.5 inch "M-203" barrel (the indentations are for mounting the M-203 Grenade Launcher), retractable multi-position stock (evolved from the XM-177/CAR-15), 3-position fire selector, available in flat top or fixed carrying handle versions, may or may not have any of the various Picatinny rail systems or a standard forearm.

http://www.colt.com/mil/M4.asp