Andy,
The MP5 CH never bothered me. The short receiver places it in a "reachable" location, and as you said its fixed. This is especially true with the K. Between the Choate sidefolder (short LOP) and above comment - I could work one quickly.

Totally agree with the G36 comment.

My only UMP experience is with a couple of semi auto 10rd USC. I never shot them enough to have any problems.

As a side my dead friend also had an Austen SMG which is basically a Sten with a MP38/40 folding buttstock.

Swinging this back into the 308 Battle Rifle OP Lane...

Have you tried any of the 308 AR10 variants or have you seen the Rob Arm XCR-M? I had one of the first XCR-M and if the company had a better rep I would have kept it. I think it had some GREAT design features, but it was HEAVY. I sold both of my Colt LE901-16S Mod Carbines earlier this year. Outside of being a 308 (benefit) it was just an extremely heavy AR (detractor).