Atomic Annie. Technically, it's rifled and made for battle. I bet I could pull the trigger myself. Just make sure you DUCK!
Actually, for singular combat I'd prefer something like a Blaser 93 in 338 Laupa. I want to be able to reach out and touch someone and do it while concealing myslef. Attach a good 20x scope and a suppressor. Maybe a good tripod as well. I'd be screwed in a firefight, however.
In a Jungle firefight, give me a Sig 55x series gun, the carbine model. Perhaps a Belt-fed MG like the Minimi.
In house-to-house combat, I'd have to go for the G36C. Light, durable, and maneuverable.
Hedgerows, give me an FAL or G3 any day of the week.
In the Desert, I'd prefer something like the AK-47 and AIR SUPPORT.