A brief treatise on upper lapping.
Very poor practice IMO.
Unless it is a cheap assed $25.00 upper from SARCO or the like I have yet to see it make a tangible difference. In all of the AR platform rifles I have built I can only recall 2 instances where the receiver ring was far enough out of perpendicular to the receiver centerline it would have made any difference. In both instances the uppers were replaced with known good uppers.
Any error on an upper should be easily compensated for by dialing in the optic.
As a matter of fact I feel it does more harm than good.
The only thing keeping the steel of the barrel extension from contacting and for all intent and purpose battering soft 7075 aluminum is the hard anodized finished of the receiver ring face which is removed by the lapping. Even though the barrel nut holds the barrel extension tight against the receiver nose (for now) the repeated recoil impulses can cause plastic deformation of the receiver nose resulting in a loose barrel.
That .003 of anodizing is essentially a "case hardening" for the aluminum and you just took it all away.
We also need to look at it from a Metrology standpoint which is critical when truing mechanism to concentricity, perpendicularity and axial alignment.
The lapping tools I have seen are at best a slip fit in a receiver with a +- .003 tolerance (IIRC) allowance on the bore. Impossible to obtain true perpendicularity with that kind of allowance.
The only way to to obtain an absolutely perpendicular and square ring face is to use the barrel "socket" surface of the receiver as your datum as this is the true registering surface for the barrel. One would have to have a mandrel and a series of bushings in .0005 increments to accurately "dial in" the bore centerline similar to dialing in a bolt action for truing.
Waay too much work IMO for negligible gain. I have done a few but have not seen a measurable difference.
If you want to invest some time in something that will make a known and proven improvement bed the barrel to the upper or just LocTite or Devcon the barrel to the upper.
On an accuracy rifle the barrel and upper should be treated as a disposable unit.