Questions about my first AR build???

[Tedward]

All this talk about squaring up the extension to the upper is really deceptive. If the extension is installed on the barrel by the barrel maker that should be square. Then you put the barrel into the upper and when the barrel nut is snugged then torqued, it presses the extension flat against the upper receiver. There is no adjustment. If you find it off by the method LRRPF52 (Midway) said with the dykem to see the contact points, then using the compound is just to smooth and make it contact evenly.

Is that all your talking about doing? Mate the surfaces of the back side of the bolt lugs to the barrel extension with the lapping compound?