Hello. As the title states, my G&P M4 keeps jamming. I clear the jam, and i can shoot off a few bb's and it will jam again. I have tried three different hop up units (Two G&P and a G&G) and three different barrels (G&P, G&G, and Matrix), two types of BB's (Javelin and Elite Force), high-cap and mid-cap magazines, even two different outer barrels, yet the problem still persists. One of the first instances it jammed was during an Op, and I kept firing the gun to try and clear the jam (not smart). Then my gun wouldn't fire and I opened the gearbox to find my piston in two pieces. I replaced it with a G&G piston I had. I put the gearbox back together and it cycles properly. However, the problem still continues. I am honestly at a loss as to why it keeps jamming. I honestly don't think it's the hop-up unit, as I've tried so many and I don't think it's the BB's either. I believe the problem lies in the gearbox. Is it possible the nozzle is not cycling the BB's properly? Should I ship it out to someone to be professionally repaired? I really have no idea what to do. Any help at all would be much appreciated.