Before you get too excited about truing up the receiver,,,It might pay you to check and see if it really needs it...
Place your barrel in the receiver and visually inspect it to see if there are any gaps between the flange on the barrel after the barrel nut has been snugged down...If you can find some very thin thread, see if it disappears when wound around the joint and that would indicate that it needs 'truing'...If you want another way, apply a very thin layer of Vaseline to the barrel flange and see if it has a consistent transfer to the upper when the barrel is inserted in the upper...Just insert it straight in and don't turn it..
I have an older DPMS that has been set up for competition with quite a few upgrades and I have an upper that I bought at such a discount that I'm reasonably sure that nothing extra was done prior to it's assembly and it shoots almost a good as my competition rig...Maybe 'luck of the draw', but I've never felt the need to change any of it...
