- The Umarex H&K HK45 Co2 Non-Blowback airsoft pistol is a Co2 airsoft pistol with a fixed slide. The fixed slide means less wear and tear on the internals and more time shooting. The HK45 Co2 non-blowback pistol also comes with a 15-round magazine and 20mm accessory rail. This is an Officially licensed replica with Heckler & Koch GmbH Trademarks! We recommend our Valken 12g Co2 cartridges for use with this Umarex airsoft pistol.