I *always* engrave the lower since the ATF considers the lower to be the firearm in an AR platform. And engraving the upper on an AR would really put a bind on being able to resell it at some point in the future. Engrave the lower and then swap out a whole host of barrels at will without issue. Engraving the upper: You are basically making something that is not a firearms into a firearm with very expensive transfer requirements. I'd love to know the *name* of the person who gave out this directive...