What's the best caliber choice for a UMP conversion? I know the 9mm is hard to find and there seems to plenty of .40's , .45's. Which would be the most accurate out of the UMP 8 incher and fun ( I heard the .45 USC is uncomfortable) to shoot as well as most desirable if it was ever sold? I'm talking semi auto fire only, so any UMP owners feel free to chime in. Looking to do a future purchase.