Receiver Lapping??

[LRRPF52]

I have built about 20 AR's of various persuasions and started lapping after one of 'em was sufficiently off-square that I could see the barrel angle. Most have been"pretty close" but none have been truely square. Also note that I will only use forged uppers.
There is another consideration, I picked up some ceracoated uppers with the coating applied to all external AND internal surfaces. When I did a first pass "do the pieces all fit?" there was some interference between the bolt and the coating inside the upper. Using the rubbing compound with the tool on both the length of the tool shaft and the barrel mating flange it cleaned it up real well. Note, when doing this, I mount the upper in a vice vertically so that there is no lateral force to the process.

You bring up some good points that I have seen over the years.

First off, Cerakote H Series is not meant to have metal running along it, and it will gall as well as cause significant coefficient of friction on the carrier rails if applied inside the carrier raceway. A true Mil-spec AR15 will have dry film lubricant baked on the inside of the upper, which helps suspend lubricants.

I personally prefer not to coat the inside of the upper when Cerakoting. The lapping tool, as you say, is a great way to take that inner layer of Cerakote down if it's there.

I also do the work in the vertical axis, as to not let gravity bias the tool in one direction, after consulting with an engineer/gunsmith friend who is one of the most exceptional smiths in this industry.