Heckler & Koch USP Blowback CO2 Powered 6mm Airsoft Pistol with Threaded Muzzle The real deal officially licensed Heckler & Koch USP CO2 powered blowback airsoft pistol. This HK USP uses a single 12 gram CO2 cartridge to power its was through mag after mag of BBs, with velocities in the field safe 330 FPS range. The magazine holds 18 6mm BBs per full load. This USP is ready to go to work out of the box or with the muzzle accessory of your choice.