Just got back from a 1 day carbine course. It was a lot of fun and a great learning experience/ motivator. First thing this morning the instructor had us confirm zero. When I went back to my gear to top off my mags my receiver extension rotated as I sat the rifle down. I looked closer and noticed the castle nut was trying to back off. Good thing I keep an AR multi tool in the range bag. My day was almost ruined. It would have been embarrassing as hell. Thankfully it was caught before backing off to far sending springs and detents went flying. Moral of the story, STAKE. Oh yeah, take a damn class. You'll learn a lot about yourself and your gear.