co2 leak?

When I put the CO2 Cartridge in the pistol the CO2 leaks out of the Barrel and in the handle where the point where the CO2 cartridge is punctured. I have tightened the screw at the bottom that tightens the cartridge into the point where the cartridge is being punctured. Any Ideas? I have taken the pistol apart and could not see anything amiss. Everything seems to work just great accept the CO2 leakage. It drains a cartridge pretty fast.