If we are just talking about real guns here, I'd go for the M4 A1 Masterkey. Loads .223Remington ( or 5.56 NATO ) over, and 12 gauge shotty under. 30 rounds per clip and 7 shottys in gun. My fave in Zabul province. Also another fave over there is the .50cal Barrett heavy interdiction weapon. Prime boomboom.
and the CTARs kinda like the famas and a gun that comes with a reflex sight would be sweet
and yeah the f2000 does look kinda like the MA5B in Halo