I've used a lapping tool for upper receivers that I bought from Brownells. The lapping tool was easy to use - I've lapped two receivers so far. I got my coyote rifle (chrome lined 16" HBAR) to shoot 1.2" 5-shot groups with handloads and factory ammo (Hornady TAP). I don't have accuracy results for the second rifle yet (6.5 Grendel).
AR-15/M16 UPPER RECEIVER LAPPING TOOL | Brownells
This lapping tool was highly recommended by an AR-15 gunsmith friend who swears he can get most AR-15s to shoot 1" or better groups. This is one of the tricks he regularly uses. He said he gets similar accuracy with both SS and chrome lined barrels, BTW, rarely does he have to replace a barrel to get an AR-15 to shoot well.