Kwa m4a1 problem
 

Hello, I just bought a KWA M4A1 and as soon as I started playing fully automatic, a problem happened. The motor is spinning but the gun is not shooting. I think there might be something wrong with the piston beacause it's not slaming. Can you help me please?

sounds like your motor isnt contacting your gearbox. just adjust it by tightening the screw on the bottom of your grip (the big one right in the middle)

Busted piston or stripped gears. The gun new or used?

The gun is new, I know it cause its my friend that asked the question. He fired maybe 1500 bb's in semi auto and tried full auto for 50 bb's before the gun started to make a "twiiiirlllll" sound (motor spinning)

Actually I tried to thight the little screw under my grip but as soon as I put the battery back in the fuse blew up. Maybe I turned the screw too much?

The gun was new I bought it on Buyairsoft.ca

Buyairsoft? There's your problem.
But it sounds like your gears or piston is stripped. And yes, you did turn the screw too much. You've tighten the motor to the point where it couldn't turn over so it blew the fuse.

Also doing 50 rounds bursts are not good for your gun, thats for sure. 3-7round is recommended.

so what would you recomand me to do? send it back and try to get a new one or try to fix it with your help?

I'd dought you'll be able to return/exchange it since it's been used. You're options are to learn and fix it yourself or take it to a gundoc or someone with experience with internals.

Thank you very much for your help the last thing I need to know is where can I find a gundoc around montreal?

check the gundoc thread.