Castle Nut Staking
One of the little details to look for in an M4/AR rifle is the "staking" of the castle nut on builds using receiver extension tubes. Staking prevents the nut from backing off under hard use. If that happens you may find your stock turning when you don't want it to.
Staking is fairly simple. Once the castle nut is tightened to the appropriate torque, take a punch and tap some of the metal from the receiver end plate into one of the notches on the nut. If you need to remove the nut in the future you will have to punch the little tab of metal out of the slot and use one of the other staking slots on the nut when you put it back on.