what can be the problem every time i insert the C02 cartridge and screw in the piercing knob all the CO2 leaks out what could that be thanks.

I would first remove the 3 screws that hold the stock. Remove the stock. Then you will see an access hole. Look through the access hole, and examine the co2 catridge seal. All it takes is a little speck of dirt, and it will leak. Make certain the seal is clean, free of debris, and in good shape!