Necro thread resurrection... Because this is what I'm doing to my UMP.
Base platform: UMP converted USC with my ACR stock conversion block, Navy conversion kit with UMP lower with complete trigger/hammer/sear swap, milled the upper here at my house, had Tommy Bostic do the bolt milling on a factory UMP .40 bolt, SBR'd and suppressed.
I bought a second .40 barrel off Gunbroker to keep in reserve because my USC/UMP is SBR'd and BATF says I can change calibers but I must ALWAYS have the ability to switch back to the original filed caliber. My original .40 barrel I had threaded 9/16x24 in front for a suppressor which has worked quite well also. I'm running the original .40 UMP BCG (with the heavier tungsten insert still in it) and it fires just fine once we reamed the chamber for the longer 10mm round.
One thing I found is that yes, the chamber is fluted, but we're only adding .30" of unfluted chamber deeper into the barrel and right where it meets the shoulder, so it shouldn't be and was not a problem. Extraction functions fine, bolt locks back on an empty mag just fine, that's not the issue at all. Recoil feels about the same as it was with .40, but granted that's with 10mm 185gr JHP from Freedom Munitions, not the REAL 10mm loads from Buffalo Bore or Underwood (loaded a lot hotter).
Originally the idea was that I'd also be able to shoot .40 in it still because I can in my Glock 29, but the issue is in how the round is held in the Glock - it'll work in a Glock but with the way the UMP bolt works, the .40 bullet has nothing to keep it from dropping all the way to the chamber shoulder so that will not work in a UMP.
The issue I'm running into is mags. The UMP .40 mag isn't anywhere near deep enough for the longer 10mm round. I bought several .45 mags but the problem with THOSE is that the upper retention / feed lips are a LOT wider for the wider .45 casing, and they won't retain the 10mm round, it just pops out the top. I tried just one round carefully placed in the mag and it fed OK, won't know about mag spring pressure until I get the feed ramps right.
In comparing mags (I can't find the other referenced thread UMP Mags wtf) it appears there is a plastic blocking tab that runs down the length of the magazine in the .40 caliber that keep the bullet from seating as deeply back. If that were removed, the 10mm round would fit. I may try that first, as the alternative is taking a Bondic pen and adding plastic to the lips of the .45 mags then filing them straight.
Just bookmarking brainstorming ideas and seeing if anyone else has actually done it with ALL parts from UMP's (not grease gun lowers). I'd like to keep the ability to also run .45 still in those mags just to have every caliber still covered (I have a 9mm kit as well), so I'll have to be careful with adding material (although I can always just file it back down).