03-12-2014, 03:42 AM
Some on this forum and elsewhere on the interwebs have questioned the ability of the lapping tool to square the receiver face to the bore.
It has been noted that the lapping tools are cut to 0.997" and that the receiver bore can be as large as 1.002". That leaves 0.005" of clearance that the tool can be off axis.
The 0.997" portion of my wheeler lapping tool is 5.5" long. Imagine the tool sitting in the receiver in the worst off axis position. The tool would contact the receiver bore in two places. Connecting the contact points would create a right triangle with a 5.5" leg and a 0.005" leg.
atan(0.005/5.5) = 0.052deg or 3.13MOA.
3 MOA isn't that bad. Who has taken a brand new upper and mounted a new scope to it with the windage in the center of its adjustment range? I have and I was off by more the 3 MOA.
I know there are other tolerances in play, but the worst case clearance won't add a whole lot of error.
Some important things to note.
1. That angle only puts the bolt lugs off by 0.00067". There should be enough wiggle in the bolt and carrier to overcome that gap and get even lug contact. (Just and educated assumption on my part.)
2. Even though the receiver face is non-perpendicular by 3MOA, the receiver face is very flat. The flat face will keep the barrel from rocking back and forth on two high spots.
3. It is possible that the tool will self-center a little bit with a heavy oil film.
If I made a mistake, don't beat me up. It is late and I graduated college a long time ago.
I will post a sketch soon.
It has been noted that the lapping tools are cut to 0.997" and that the receiver bore can be as large as 1.002". That leaves 0.005" of clearance that the tool can be off axis.
The 0.997" portion of my wheeler lapping tool is 5.5" long. Imagine the tool sitting in the receiver in the worst off axis position. The tool would contact the receiver bore in two places. Connecting the contact points would create a right triangle with a 5.5" leg and a 0.005" leg.
atan(0.005/5.5) = 0.052deg or 3.13MOA.
3 MOA isn't that bad. Who has taken a brand new upper and mounted a new scope to it with the windage in the center of its adjustment range? I have and I was off by more the 3 MOA.
I know there are other tolerances in play, but the worst case clearance won't add a whole lot of error.
Some important things to note.
1. That angle only puts the bolt lugs off by 0.00067". There should be enough wiggle in the bolt and carrier to overcome that gap and get even lug contact. (Just and educated assumption on my part.)
2. Even though the receiver face is non-perpendicular by 3MOA, the receiver face is very flat. The flat face will keep the barrel from rocking back and forth on two high spots.
3. It is possible that the tool will self-center a little bit with a heavy oil film.
If I made a mistake, don't beat me up. It is late and I graduated college a long time ago.
I will post a sketch soon.