Hi len, everyone is correct that is an H&K UMP. You can get a civvy version called the USC, they both have skeletonized buttstock but no flash hider, just a long barrell chambered in .45acp. The early versions came in grey but you can dye it black. The new versions are black now, if you buy an H&K MP5 clone from like Cohairie arms, they all come with fake silencers to make the rifle the legal 26 in over all length. Anyway, just look on Gun Broker or Guns America, type H&K USC on the search box, I like Guns America.com better for auctions, less people use it and thus the auction price is kept lower. High Caps are also hard to find for the USC and very expensive as well. I suggest getting the MP5 clone, the ammo is cheaper (9mm) so you can shoot more of it affordably than .45. You can even mount the surefire flashlight on it for that tacticool factor. I have a Cali-legalized version of it with hi caps I had lying around before 2000 and it rocks! Cheers!
A guy told me one time "don't let yourself get attached to anything you are not willing to walk out on in 30 seconds flat if you feel the heat from around the corner"