Polarstar UGS CO2 Insert - 12g MilspecSpecifications: Brand: Polarstar Compatibility: UGS Universal Gas Stock system from PolarStar Construction: Aluminum Brass Features: Equipped with integral expansion chamber, anti siphon system, 5 micron filter and check valve to prevent liquid CO2 from entering the regulator regardless of the rifle's or air rig's orientation Also prevents gases from escaping the expansion chamber while switching out cartridges so as to conserve as much CO2 as