I don't know about you other G&G UMP owners but I've found trying to acquire low/mid cap mags to be a very difficult process. Anyway I noticed that the CA UMC mags looked very similar to the G&G hi cap mag I had so I got Jugglez (who is now a CA dealer woo!) to order me 5 and yeah they work!
The CA UMP mid cap mags (100 rnds) fit a bit tighter than my G&G Hi cap mag but it does fit and the mags feed with no issues (both semi and full auto) with my gun. Just wanted to put it out there to other G&G UMP owners who are tired of running the hi cap mags.