What type of head is on the screws? Slot Screw driver, Allen, or the new torx. The first step I would try is to get a tight fitting screw driver handle type tool with the proper head, vise the gun down and insert the screw driver, while putting left hand pressure on the screw, tap firmly on the driver handle with a mallet. If this doesnt move them, the next step would be to drill the head with a bit till you make contact with the ring, as soon as the head is drilled off you can use pliers to remove the end of the screw. I have even used a drimel with a cutting disc and gone between the ring halves and cut the screws.
