CO2 ctge expels as it is inserted into handle (by 2 users)

Your first problem may be your seals. if the base seals have not been oiled since you bought the gun they may have weakened or cracked to the point that they no longer make an effective seal against your 12g cartridge. For now, add some gun or machine oil. If that fails you can buy seal kits from whoever made your gun, and possibly have your local gunsmith install them if he is able to. This all depends on what the gun is worth to you.