I bought a used Sig Sauer MCX from a local pawn shop. When I insert a CO2 cartridge, the gas leaks out in about 20 minutes. Examining the port, I cannot see an o-ring to seal the cartridge neck even though there is what appears to be a groove at the top of the threads for one. Can anyone tell me where I can buy a replacement or what the specs for the o-ring are so I can search for one?