Gas airsoft pistol mag leaking?

i recently bought a licensed smith and wessonM&P40 from evike.com and i shot 8 gas cartridges worth of bbs i put in a new 12g gas bottle and screwed it in to where i hear the fizzle then it stopped like usual but then the gas sprayed out of the back of the magazine where the plastic meets the pump i tried it with 2 more gas cartridges but the samething happened so i googled things but i couldn't ind anyanswers so can u help me buy telling me how to fix it or at least whats wrong because my 14days have past for the evike warranty and the manufacturers keep putting me through a loop