I have put together a couple of dozen mongrel dog AR's... and quite a few quality parts AR's also... When you drop the bolt carrier for the first time... you can almost tell if it is going to work right out of the bucket, just by the sound it makes when closing! I have narrowed it down to the gas key to upper fit...  I know that most of you guys that build your own, know what I'm talking about... any other ideas??  till..later...GT

There is a subtle but distinct sound made when the bolt properly engages and rotates fully into the locking lugs.  I listen for it.  Especially when dealing with live ammo.   If that sound isn't right with a properly sized dummy round (or a live one) in the chamber, the headspacing isn't right, either, or at least I check it with a headspace gauge first.

CJ