it pulled the spring in semi auto but on full auto when the piston was left in the middle after firing the motor could not pull it the rest of the way , the battery is just fine and is the biggest mah that fits my gun , i know its not the battery because i tried it with a 10.4v 3600 battery and it still happend , and this is not the first time i,ve worked on a gun i know how to shim the gears. with the original spring it's working great , not the fps i want though. i don't like stores that don't take back merchandise . i would have bought the systema motor for 90 but he said the ca one for 48 would work , it won't, bad advise.now i cant exchange it so i'm stuck with it and can't get the systema motor. i would gladly have exchanged it and gave the 50 extra for the other motor, but they dont refund or exchange.