Telekinesis said:
I'm sure it could be done, but it wouldn't be cheap. You'd probably need to start off with a USP in .45, and then have a custom shop convert it to 10mm. I believe a group from Sig Forum got a bunch of people together for a group conversion of 220s from .45 to 10mm a few years ago. That could be a good place to start looking if you're serious about getting a USP in 10mm.
It's not that huge a deal. You could start with either a .40 or a .45 but personally I think the .45 is more work.
Starting with a .40:
Ream the chamber to 10mm with a finish reamer. Put stronger recoil springs in it. This is where my .40 is right now. It'll chamber and fire ONE 10mm from a .40 mag. It'll extract and eject the empty. Sadly the COAL is too long for a 10mm to be loaded in a .40 mag in any position except the top one, so there's more work. The Ejection port is also too short to eject a loaded 10mm round. So you can either mill out the ejection port and refinish the slide, or drop the mag first and unload through the mag well. As far as mag choices, I have found two. The 10mm round will fit in a USP .45 mag, but that mag is just a touch too large for the magwell of a .40. (you can jam it about 2/3 of the way in before it gets suck.) So you
could very carefully take about .005" off each side of the inside of the magwell and a tic over .010" off the rear face. It
lookslike there's room to do that without getting in the way of the main spring. You have to leave the front wall of the magwell alone because that's where the mag catch is. Also if you take too much off the rear wall the mag catch won't engage the front of the mag. After you open the magwell you should be able to load 10mm in a usp45 mag and fire it through your (previously) usp.40. The mag will protrude a bit from the bottom of the grip but it looks as though the feed lips will be at the right height. That's probably the easiest way, especially if you don't open up the ejection port.
I didn't like playing with the frame because I'm trying to make one I can switch back and forth. so Mag option 2: (Note: I started this in project in the early part of 2013 when Mags were unobtainium, so I haven't tried any of this part myself yet) I'm told that a STI 2011 mag will fit in the magwell of a USP. So if you add a divot in the front to grab the mag catch it should work just fine. STI also makes 10mm 2011 mags so the feedlips should be better (narrower) then a .45 mag. The STI mag will stick out farther and the baseplate will be at an angle other then horizontal, but that should be fixable with some careful mods to the bottom of the mag. We'll see when I have mags and myself in the same state at the same time. Personally I think the modded STI mags will be the way to go.
If you want to start with a USP .45, obviously the mag problem is solved, but you will need a whole new barrel either in 10mm or in .40 that yo ream out yourself. And the breechface will be too wide. You will either need to weld up and re mill the breechface (and move the ejector and extractor closer together) or deal with the rounds having too much lateral play in the feed/extraction cycle. I *think* you'd find that having a wide breech would lead to reliability issues in the converted firearm. Also if you change the breechface you'd have to fit the new barrel to whatever changes you had made. Overall there's a lot more careful machining in this option.
There's some threads and pics floating around the internet of others that made 10mm USP's successfully. I think it will be cool when finished. I would have just bought a factory one if it was offered though.
Also FWIW it was Bruce Gray over at Gray Guns that converted the Sig220's to 10mm. I'm sure he could make a USP for you, but he's a talented smith, and talent isn't cheap.
