I've owned a USP for a while now and have widely proclaimed it's jam-free nature... until three weeks ago. Since then, it has been jamming every 50 rounds. The problem is a failure to eject. The extractor doesn't seem to be holding onto the brass on the cases that get stuck.
I changed ammo, but it still jammed. I called H&K about the rounded bottom of the extractor and it's supposed to be that way (guess I never noticed).
Then I thought about all the gunk that I clean from the outside of the ejection port and how some of that probably edged its way into the extractor. I took the extractor out and found at least 1/32" of powder residue compressed between the extractor and the frame. I cleaned it up and reinstalled it.
As of last night, 300 rounds and no FTEs. That doesn't prove anything yet, but 300 rounds of mixed ammo is an excellent sign.
So, my recommendation to USP owners is to clean your extractor out before the gun hits 25,000 rounds. You might want to check the firing pin too, but that's another story.