I was shocked when I got the SMG and it did not use pistol ammo like in real life. Nothing like firing an 9mm AR SBR (Short Barreled Rifle )that is suppressed to get the blood pumping. I would build one but that is 2 $200 USD Tax stamps ( one for the can and the other for the SBR) before I even get into the actual build and add an an red dot to it. This is what I would build it on . http://palmettostatearmory.com/psa-glock-9mm-complete-classic-lower.html with a 10.5 inch barrel and go with one of the 50 round Glock drum mags. The Colt 9mm AR mags just are not reliable and they are hard to find anymore.