AGM MP40 MP007 airsoft gun not working?

I recently purchased a AGM MP40 MP007 full metal airsoft gun from my friend for $70, and it was working great at his house, and then at mine, but all of a sudden it just stopped firing while I was firing it on full auto. I finished a burst and then was about to open up for another one and it just wouldn't fire. I don't know what's wrong, but the piston chamber door (I think is what its called, its that metal thing with the piston in it, might be the gearbox sorry I'm a total noob) is open when the gun isn't firing (which might be normal) and I have the front removed. Anyone have any suggestions?