Umarex HK Heckler&Koch MP7 Navy Gbb Airsoft Rifle. The HK MP7 Navy has a hard-kick blowback design that operates on Green Gas with a 390 FPS muzzle velocity. A top optic rail along with rails on both sides and the bottom of the barrel allow infinite loadout combinations. The collapsible metal stock allows operators to choose between two- or one-handed firing stances. Full / Semi Auto. Powered by Green Gas. Umarex HK Heckler&Koch MP7 Navy Gbb Airsoft Rifle Specifications. Caliber: 6 mm BBs. Barrel Length: 7.5. Length: 16.5″-25.4 “. Weight: 4.3 lbs. Shot Capacity: 40 rds. Action: Full/ Semi Auto. If the wrong contact details are provided any losses incurred as a result will be the buyers responsibility.