Primary Weapons Systems. You may have some trouble getting a 7.5" 7.62 x 39 barrel to run smoothly. Don't plan on running it suppressed that short unless you really open up the gas port in the key and barrel. I've done quite a bit of research on SBRing this round and it seems that PWS is the only company that has perfected an upper and it's not as short as 7.5".