USP 10mm Conversion Details & Range Report

[Pinion327]

I know that this report is a little over-due, but better late than never. I was hoping to include some pictures with this but I'm having some trouble with my photobucket account, so those will have to come later.

Ever since I bought my first USP in .45acp nearly a decade ago I've wished that HK would offer one in 10mm, which (in my opinion) is one of the best handgun calibers ever developed. I always thought it was odd that they didn't, both because the USP platform is a natural for the hard-hitting load, and as a complementary sidearm to the MP5/10. But it wasn't meant to be, so I had to content myself with G1ocks...

Then I discovered the HKPro forums and the mysterious "search" feature. After hearing of a few other successful conversions and talking with BIG_CHRIS_304, I decided to join the club. And in an effort to help those attempting to do the same thing I'm going to try and make this as "rookie friendly" as possible, so please forgive me if am overly descriptive...

I started off with a barely used USP40 in stainless steel. Even though the HK Hostile Environment finish is harder and stronger, I went with the stainless in case I needed to make any modifications to the slide later on. The conversion package that I purchased from BIG_CHRIS_304 included an HK factory .40sw barrel that had had the chamber reamed out to 10mm specs, a 16lb. recoil spring, and a 12 round modified STI magazine.

One of the tricky parts here is the USP's captured recoil assembly, which makes swapping springs a significant (expletive deleted) task. To deal with this I bought an early production (KE dated) non-captured guide rod from MaverickH1 (thanks Mav!) to use instead. The spring you end up using is going to depend on the power of the loads you intend to shoot. The 9mm and .40sw USPs both use a 14lb spring. While you might be able to get away the factory spring if you are only using VERY light 10mm loads, you really should be using the 16lb. spring. And besides - if you're going to go 10mm, what is the point of using FBI-lite loads anyway?

In my range tests I found that the 16lb spring did a great job with loads ranging from the UMC 180gr (vel. 1150, en. 529) on the low end, and up to the relatively hot DoubleTap Match Practice 180gr (vel. 1250, en. 625) and Winchester Silvertip 175gr (vel. 1290, en. 649). I also tried several of the very full-power DoubleTap loads in 135gr (vel. 1600, en. 767), 155gr (vel. 1475, en. 748), 180gr (vel. 1350, en. 728) and 200gr (vel. 1275, en. 721). I did not have any feed issues with any of these loads, nor did I see any signs of danger when examining the weapon between mags. However, I would recommend using an 18lb spring with any loads that are over 700 foot pounds of energy for the sake of offsetting stress on the frame and on your wrist.

This is easier said than done. I was physically unable to change to the 18lb spring at the range, and was only able to do so at home after about 30 minutes of struggling with the thing. What worked out well on the next trip to the range was using the 18lb spring in the captured recoil assembly for the hot 10mm loads, and using the non-captured to swap out the 16lb spring for the low to midrange 10mm and the 14lb spring for .40sw. FYI – I also suggest using a 16lb recoil spring with the some of the super hot .40sw rounds available from DoubleTap and Georgia Arms, as they are just as potent as many of the 10mm loads out there.

The next item on the agenda is to acquire more magazines. The STI mag that I got from BIG_CHRIS_304 works great and I love…but I must have more! Because I mainly use the Jet Funnel attachment when shooting 9mm or .40sw, I’ll have to find or make some JF mags in 10mm. Of course, because of the 10mm’s reputation as a game round I’d like to put together a hunting setup with a six inch barrel and a scope mount. Also, I may at some point open up the ejection port just a little because as it is it’s too narrow to eject an unfired 10mm round, but I’m not in any hurry to do so at the moment.

All in all, I’m very pleased with the conversion. Special thanks are in order for BIG_CHRIS_304; you did great work and it was a pleasure to do business with you. Finally my passions for HKs and 10mm handguns have come together. And now - at last – I can have my cake and eat it too!

I hope all this helps…Feel free to contact me with any questions or advice.


P.S. The bullet velocities used above were taken from information provided by the respective ammunition manufacturers.