03-12-2014, 12:15 PM
dmsims21 Wrote: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.
Interesting,
Here is my take. I think it would be impossible to true the face perfectly. When I test fitted the barrel on the 300, there was a noticeable difference on the barrel centering with the 13 inch rail, it was obvious the barrel was not centered. The 13" rail being so long made the run out so obvious. The upper receiver was a Surplus Ammo upper, supposedly machined by Aero Precision. The barrel extension was a very tight fit into the receiver. The lapping tool to upper fit was also tight, but with a light coat of oil is was turning fine with little bind. After the lapping the barrel was perfectly centered in the rail. When I sighted in the rifle, I used the red dot off my carbine. The carbine was pretty much centered when compared to its rail, and the rear sight was centered for windage. The 300 was only off windage wise 1 MOA. When considering the variance in the top rails, and a cheap mount that was not designed to re zero I think it was pretty good.
I think you are right, what it comes down to is minimizing the run out on the assembly as a whole. Provide a flat surface for the barrel extension to seat against. Im not an engineer, just a mechanic that is use to taking measurements and making sure of proper clearances and runout on some pretty high tech engines. I could be wrong, but on a receiver face that is not true, I suspect the lack of clamping surface area on the barrel extension would induce some harmonics into the barrel when fired. I could be wrong, someone else here may know better than I do. As far as I see it, there is really nothing to loose by lapping the upper face, and a whole lot more to gain.
One of my early mentors that use to build some amazing race engines had a big secret. Many guys built the engine to produce the most power, to win races. He would do everything he could to reduce parasitic drag, and heat without hurting reliability. Even at the expense of horse power some times. He told me if you can reduce these things without hurting performance, there is no reason not to. If a little bit of reliability costs a horse or two, then its worth it. HP does not win races, when you are broke down half way through.
The only draw back I can see with lapping is the removal of the hard coat anodizing, the benefits seem to out weight the draw backs for sure.