First thing I do is give it a blast of carb cleaner, followed by compressed air. Once the nut is broke loose, unscrew the receiver. Then blast the nut with more carb cleaner and air while unscrewing it. As long as you are screwing the nut away from the muzzle, the shot is left behind just in the gap of the relieved threads in the nut. Once you get them apart, inspect the barrel threads and the threads in the nut. You may have to use a dental pick to remove some that are stuck.