The Colt LE6920 I've got now had the castle nut staked on there pretty good and was a total pain in the #### to get off.
OP, I didn't think I'd ever need an AR wrench, and the first time I needed one, I ask to borrow one from a buddy. I figured I would ever only need it this one time, when am I ever going to remove the rear plate again? Well, I found myself needing one again, and again, and again.
They're only $50 on Amazon, just go buy one......warning though, after you buy one, you'll start finding excuses to use it....like building AR after AR after AR......