Hope you don't mind me piggy backing on your thead, have been tinkering with the USP lately too.
Any idea how to remove the magazine follower? Is the little knob threaded or pressed in? I want to address the sticky follower, but don't want to destroy the thing and it's the only part of the marker you can't really remove.
Found most of the parts to rebuild mine, FYI action village has 12g pierce assemblies right now. Found a donor all metal piranha with a double trigger frame to steal. Would prefer the single metal frame but those seem oddly hard to find.
Really want a vertical fore-grip but failing to come up with a solution at the moment.
I'm surprised these don't pop up more often for sale, even as parts/repair/broken. I bought mine at Dicks sporting goods, they were pretty common and easy to find when new.