Dr Fallout
Centurion
As far as I know the only gun that uses 5.56 is the "assault rifle" (it's really a machine gun, but I think we can safely say that Bethesda doesn't care about gun nomenclature, or having anything close to realistic weapons). The closest you can get to a Mosin Nagant is the hunting rifle, which technically uses 7.62 ammo (7.62x51 not 7.62x54).What weapons use 5.56 in that game? I don't recall finding any real weapons that used it yet. Someone should mod something like a AR-15 in the game if there isn't one already, would make use of all the 5.56 ammo lying around. Is there some weapon like a Mosin Nagant in the game too? Wait I don't think I've found 7.62 ammo so that's probably a no.
Fair enough, the hunting rifle in my game uses .308 which would be hotter then the 7.62x51 if I recall correctly but minor differences. Surprised there are no AR-10 rifles to take advantage of the .308 ammo as well. Into weapons yourself it sounds like.
Woah... too much gun talk... I have no ideas which guns use which ammo.