Semi-auto in 10mm with an arm brace would be pretty cool. Much, much better than those semi-auto AP5's or whatever they're calling the semi-auto only copies of MP5's in 9mm. Back in the late 90's long before I retired I went out to do a walk and talk with a couple of FBI agents at an apartment that was being used to house alien prostitutes (regarding international alien sex trafficking investigation). One of the agents pulled out a 10mm MP5 and the other agent admonished him because it was only a voluntary knock and talk. Pretty cool to actually see one, a real one. My agency never had cool guns like that. It had the collapsible stock if I remember correctly.