My Sniper, a JG BAR 10 has a problem. When I pull the bolt back it will briefly cock if I hold it back, but then if I let go it snaps forward and doesn't fire. I have taken it apart down to the trigger box and all the internals are brand new. Ive taken it to my local airsoft shop and he said there is nothing apparently wrong with it. Its been in one battle. Please help me!