Matt's statement about "puff" rounds (I really
like that term
) holds true for full-sized H&K USPs, too - especially the 9mm, and especially when brand new. A few guys I know have bought USP9's, largely based on my bragging about my USP40, only to have them malfunction constantly on puff (
) practice ammo the first time out. They put some hot stuff in 'em, and they work fine. They also seem to work fine with anything after a few hundred rounds.
Personally, I wouldn't put something not designed by the factory on anything I might have to trust my life to, especially for such a minimal gain. I guess if you want to do .45 Super from a Glock, it might be the only way to go, though... Sounds like a great excuse for a new USP45 or beefed-up 1911 to me!