Bruce, How much lapping effort was required to square up your two receivers?
It took about 10 min to lap the uppers. Neither one appeared to need much lapping. It took about an hour to disassemble and reassemble the .223 upper. The 6.5 Grendel was in pieces, so I only had to assemble it.
I had a Bushmaster DCM back in 2000 that would put 10 shots using 69gr. Sierra Match Kings in less than 1/2" at 100 yds. with my 18 power scope on it.
PS: I just ordered a new full length fluted 18" varmint upper from White Oak Armament for my Colt LE6920 M4 and am hopping it will shoot bug holes at 100 yds. I have already installed a Geissele SSA-E trigger in the lower.