Most enemies, monsters or not, are dropped in two shots or less in the real world. I don't think I'd want to carry an assault rifle because if there's 1000 or more, even though I'm a fairly athletic guy, I don't think I can carry that and maneuver properly with enough stamina to last. Just give me a handgun with a good clip size...
Does infinite ammo = no reload times? The bullets just kinda magically appear in the chamber? Otherwise, people will be disappointed that they have to lug around a couple hundred grenade launcher rounds.