Anyone done it? Thoughts?
When I did not know what I was doing, I did a few times. I've cleaned up a damaged receiver a time or two as well. Thoughts...waste of time and money. Only a very low quality receiver would need it, and then you are better off spending another $20 on a better receiver.
The receiver is aluminum. With anodized layer removed, it is pretty soft. The lapped area is load bearing. It transmits recoil force from the barrel to the receiver. Bedding may be a better choice, if I ever want to tinker with it.
Yes, the anodizing does create a little surface hardness. And, the portion on which the anodizing is removed is now raw aluminum. Mate that to stainless...oh no! Even on carbon steel, you will create corrosion cells that, in a little time with the thermal cycling, will create more stress risers than you eliminated making it "flat".
Best bet is buy 7075 receivers, use the proper anti-seize, and torque at the high end, 70 to 90 pounds for final. The harder extension, with proper seating by low torque with successively higher torque, will move the aluminum more than your lapping creating a rigid joint, with full corrosion protection. It is highly rare that won't get the best accuracy that the barrel is capable of. If you have a really out of spec receiver you must use, get a foil ring to use between the receiver and extension. Way better.
If you are searching for supreme accuracy, like in the 1/4 MOA range, undersized 7075 receiver or oversized extension, and thermo-fit to complete with torque in the 90 to 100 ft-lb range.
I've tested all of this with lab equipment as I was building my barrel manufacturing business up, and in consulting for other barrel manufacturers.