Ok, I did a little looking around and I found this:
Soundsnap
I read over the license. It says you can use it commercially in a "Derivative Work" (I read what they meant by this...like a movie, or game), but they can't be sold stand-alone (which I don't think you're going to do).
When your there, look up "machine gun", "shotgun", and "flame thrower." They have good sounds.
Lastly, to make sure everything's okey, I downloaded one and virus scanned it. Nothing! So, I hope you can use this (I'll probably use it in the future).