Ok it is taking the place of the charging handle in the rear of the receiver. It needs to be removed to pull the bolt carrier out of the upper. Yes a little bit of gas comes out the back that's why there is a "Gas Buster" charging handle. I would find a plug the proper diameter and threads that takes a torx bit so I could screw it in flush. Locktite it in but removable with a torx bit. Or grind off what you need to get it to close. But a plug flush with the rear would be cleaner looking.