The H&K USC carbine is the civilian version of the H&K UMP sub machine gun. The USC has a fixed Dragunov-style stock while the UMP has a folding stock and free standing pistol grip. Apparently you can convert a USC into a UMP. Les wrote to me saying …

I recently shot a multi-gun match which required the use of a
pistol-caliber carbine and ran into two guys that were using HK UMP’s.
At first I thought they were LEO, but they said that they had just
converted HK USC’s to UMP’s by following the process below (you’ll
have to scroll)

http://www.hdps.org/htm/3.htm

The guys did the conversions themselves, and the pieces were high
quality, all HK part UMP clones in .45…

H&K USC into a UMP conversions

Very nice.

[ Many thanks to Les for emailing me the info. ]