Umarex HK Heckler & Koch HK416 A4 GBB Green Gas Blowback Airsoft Rifle (2262072). Now, you can wield the airsoft variant at your local field. This gas powered Airsoft rifle shoots at a blistering 400 FPS with 0.2g BBs. You get to feel every trigger pull with the gas blowback system. This is the perfect Airsoft Rifle for you. Great for a present! 40 rounds (BBs) Capacity. Umarex HK Heckler & Koch HK416 A4 GBB Green Gas Blowback Airsoft Rifle Specifications. Caliber:6 mm BBs. Capacity: 40 rounds (BBs). Velocity: 400 fps with. Barrel length: 16.2 inches. Weight: 7.75 lbs. Length: 31 inches – 34.5 inches. If the wrong contact details are provided any losses incurred as a result will be the buyers responsibility.