Quote Originally Posted by Robinhood View Post
This is true to a great extent. It does do something. It gives you flat surfaces that mate up to each other which will aid in improved accuracy. It is not guaranteed to improve precision. We know for that you must have everything in the same axis and perpendicular. That was in Post #2.
Yep. The very slight clearance between male and female threads allows for a tiny perpendicularity error. But the flatter the face, the less chance of vibration.

Something else that I need to mention. On my very first build about eight years ago, the ground recoil lug had a screw for a location pin. The head of that screw was about .005 taller than the receiver slot depth. So the pin bottomed out before the receiver face mated with the recoil lug. Had to grind on the screw head and file on the slot to get decent groups.