Doesnt take a seal, it seals it self as it pushed its way into the co2 cyl. the tapper makes the seal, If the gun is leaking, it is the seals inside the gun. no one makes them. the gun is 'unrepaireble'

The gun is leaking internally, and it can be fixed easily... Go to hardware store and get #6 o-ring and Teflon tape used by plumbers. Disassemble gun being careful not to loose the elevator spring that pops out of the top of the gun when you slide the innards out. The rear section of the guts has a perfectly round metal surface. Use small vice grips to unscrew the round plug from the valve section and be careful not to dump all of the valve parts everywhere. The area of the plug towards the area turned with vice grips has a corroded brown seal. Use small screw driver to chip out the bad seal, clean thoroughly and replace with #6 o-ring. Apply a couple rounds of Teflon tape, pulling off about 1/8" of width of Teflon tape so it fits properly. Lubricate lightly and reassemble. Voila... It will work as good as ever!