Check the you have the bolt alignment pin set correctly. It needs to slide into the groove at the bottom of the threads for the back-cap when you screw the bolt kit into the body. If it does not go into this slot, it will cause the bolt kit to become stuck in the body.
that implys that you have undone the grub screw too far for the back cap to come off. i had the same problem wth it getting stuck i resolved the problem by regurly cleaning and lubing all the o rings and eventually they set in and the whole bolt assembly started to come out easily. now it comes out sooo smoothly, no problem.