Check your cup seal. As I said they unthread after a few shots, and will cause burping.
play today with 2 of them. definitely need a thump screw or something for switching from mag to hopper. trying to do that with an allen in a game just doesn't work.
guns shot good. there was complaints that the ball retainer doesn't hold the balls in the mag ounce the spring is released. I said just release it after it is inserted into the gun.