Losing co2

so, i assume it is a pistol but which one i have no idea
most of the pistols would have a face seal for the cartridge
the old one would have to be completely removed the area clean before the now correct seal is installed
now if it has one of those cheap nuts that hold the seal in place it would have had to be removed first so the screen and piecing pin and seal would be tight against the valve body .
but seeing that i don't know the model i have to guess
 
is it one with the lever to pierce and close the co2?
Looks like it according to the picture on AGD


Papa1724, it sounds like you were on the right track. Most likely the seal around the piercing pin is missing. If you can’t get your hands on an OEM replacement, you may want to try an O-ring assortment from Harbor Freight or Amazon to see if you can find one that is a good fit.