AHHhhh Yessss, the famous Crosman "pita breech screw". (been there, done that).I have had good success getting it out by drilling with a bit just SLIGHTLY larger than the stripped out allen hole, And just SLIGHTLY deeper than the stripped out allen hole. This will allow you to use the shank of the drill bit to snap off the screw head. Remove the breech.There will be just BARELY enough of the screw (sticking out above the tube), to grab it with some GOOD Quality pliers and spin it out.