GLOCK CO2 AIR GUN- .177 CAL.Specification Value Caliber: 4. 5 mm (. 177 cal) Action: Semi Automatic FPS: 400 to 450 Ammo Type: Steel BB Power Source: Co2 Trigger Action: Double Body Type: Glock Primary Material: Metal Magazine Capacity: 16 Barrel Length: 10. 8 cm (4. 25 inch) Trigger Pull: Double action only and has an inner barrel that protrudes out of the external barrel. You see this on many lower cost CO2 BB pistols. I believe the barrel is actually part of the action and