Since it is bulk fill it can be filled from a CO2 tank. I have a vintage Crosman bulk fill gun that has a 1/8 npt fitting on the front of the gas tube. I fill it by standing it muzzle up, then open the valve on the tank for about 10 seconds. the tank must be vertical since you want liquid CO2 in your gun. Note: the proper way to do this is to weigh the gun before and after, but I have never had a problem doing it my way. There are videos on U-tube as well.