The M4 Tank Adapter allows you to screw during a co2 or HPA tank directly into rock bottom of the grip. The advantage to employing a co2 or HPA tank is that the incredible number of shots you will get . Most players can shot all day without refilling. it is the most economical thanks to play. Shots per source: (actual results may vary thanks to atmospheric conditions, tests performed at 72 F)

9 oz Co2 = 1700 shots
12 oz Co2 = 2280 shots
20 oz Co2 = 3800 shots
68 cubic inch HPA @ 4500 psi = 4400 shots
48 cubic inch HPA @ 3000 psi = 1920 shots