03-12-2014, 02:51 PM
I have had to use oil to even get the lapping tool to spin. Add the thickness of the oil and the .005", fractioned to .0025" gets filled circumferentially pretty quick.
A lot of handguards will show run out at the fore end, especially the longer ones. That can also be an issue of upper receiver threads not being square to the BCG raceway axis, internal mating surfaces of the handguard-to-barrel nut not being true, etc.
One smith I ran into at a local range goes full OCD on every surface he can possibly think of on AR receivers, barrel extensions, barrel nuts, and bolts. He would true the upper threads, barrel nut threads, square the barrel nut internal face that abuts to the barrel extension flange, square the barrel nut flange, etc. etc. He was actually testing a 6.5 Grendel he had built when I met him, and the first group out of the pipe was sub 1/2 MOA with a 123gr Lapua Scenar load, so I wasn't going to argue with him.
Yes, it makes logical sense that having a flat receiver face helps support consistent barrel harmonics, rather than shock-setting the barrel each time you pull the trigger with an unbalanced face, which would lead to erratic grouping.
A lot of handguards will show run out at the fore end, especially the longer ones. That can also be an issue of upper receiver threads not being square to the BCG raceway axis, internal mating surfaces of the handguard-to-barrel nut not being true, etc.
One smith I ran into at a local range goes full OCD on every surface he can possibly think of on AR receivers, barrel extensions, barrel nuts, and bolts. He would true the upper threads, barrel nut threads, square the barrel nut internal face that abuts to the barrel extension flange, square the barrel nut flange, etc. etc. He was actually testing a 6.5 Grendel he had built when I met him, and the first group out of the pipe was sub 1/2 MOA with a 123gr Lapua Scenar load, so I wasn't going to argue with him.
Yes, it makes logical sense that having a flat receiver face helps support consistent barrel harmonics, rather than shock-setting the barrel each time you pull the trigger with an unbalanced face, which would lead to erratic grouping.