I've read the pro and con arguments for these things and decided to give it a try. Fit nice and snug in my upper (DPMS LoPro) and spun freely after I put a little oil on it. Read and followed the instructions. Worked really smooth unless I got the drill spinning too fast. After a couple minutes, it had lapped through the anodizing on part of the barrel mating surface, so the upper was definitely "perfectly square." After a couple more minutes I had it to the point where there was just a faint trace of anodizing left on the low spot. I'm kind of impatient so I decided it was "good enough" and decided to install the barrel.

Got everything cleaned up, torqued the barrel to 35 lbs, and... the hole in the barrel nut was still a little of. Set the wrench to 60 lbs, pulled some more, and... still not quite lined up. The instructions that came with my hand guard said DO NOT TORQUE THE BARREL NUT PAST 60 LBS (caps in the instructions). Since I'd read about people cracking this particular barrel nut while installing it I decided it would probably be a good idea to heed that advice. I took everything back apart and honed a little more. Now the trace of anodizing was gone so I put everything back together, torqued to 35 lbs, and it was still a little off but not as bad as the first time. Set the torque wrench to 55 lbs and pulled a little but not to the click, checked, and barrel nut was lined up perfectly (as far as I can tell) with the gas tube hole in the upper.

I don't know if it will make a difference on accuracy, but it did let me install my barrel with the nut within the mfgr's specified torque range, so to me it was worth it.

I'm about to pick up the wheeler tool as well. How did the goose neck drill adapter work out?

With the Brownell's lapping tool others have held the receiver in their hand to prevent binding they got when it was in the vise. I was thinking the goose neck would minimize the binding and it could be held in a vise.

How did you hold the receiver....vise / hand?

double tap

I used their picatinny rail vise. If I didn't have that I probably would have tried holding the upper by hand instead of clamping it directly in a bench vise. As far as the goose neck, it has a lot of give and if you don't hold the drill perfectly straight it wants to vibrate. I think the fixed shank on the Brownell's tool would be better in that regard.

Thanks for that info....good to know.

do not use a clamshell as it will bind the lapping tool , don't ask me how I know