"Truing" an AR's receiver face?

[markm]

It ships standard from Brownells, so it may be a week before I get it.

[lysander]

I find it hard to believe that a brand with a good reputation like BCM, etc would sell upper receivers that far out of spec. I even find it hard to believe less regarded brand sell stuff out enough to make a difference.

And, just how far out does it need to be in order to be noticeable?

If the rear sight is off by half the distance from the center to the extreme left or right, about ¼ inch, how canted is the barrel? That’s an easy math problem; the angle is the arc-sine of the windage error over the sight radius. For a 16 inch barrel and a .25 inch offset, the angle is .89 degree.

So, that means the front face of the upper receiver must be at the same angle, and by applying the same trigonometry as above, the front face would have to be out of square by 0.018”. That’s over 1/64 of an inch. You can see that much out-of-squareness with you unaided eye.

But, the barrel extension is constrained by two things: 1) the front face of the upper against the rear face of the extension flange, and 2) the gap between the inside diameter of the bore of the upper and the outer diameter of the extension. Let’s assume the front face of the upper receiver is off square to the bore by the .018” noted above, does that mean the barrel can be canted that much?

Well, let us see.

The bore of the upper has a maximum inside diameter of 1.0040 inch. The minimum diameter of the extension is .9979 inch. The maximum gap is 0.0041 inch, with the minimum distance between the contact points if the barrel in canted to the maximum is .9850 inch.

Again applying trig, the maximum cant of the barrel inside the bore of the upper is 0.2385 degrees. So, the maximum rear sight error in this case is .0670 inch or two and one-half clicks . . . .

In order for the barrel to be off by more than 5 clicks of windage, the upper has to be out in two critical places, the bore itself needs to be over size by about twice the allowable tolerance, and the front face has to be out so much you can see it.

This leaves you with two choices:

1) You are buying really poor quality uppers.

-or-

2) It’s snake oil. (unless you use it for timing)

And just another thing, when the barrel nut is torqued the clamping force is between 1700 to 4500 pounds. Given the small surface area over which the clamping force is applied, the aluminum face is crushed anywhere from 0.0005 to 0.005 inch, about the same amount as the front face perpendicularity tolerance.

To make a long post short: unless you buy crap parts, lapping is unnecessary (unless you use it for timing).

If you use Loctite in between the upper bore and the barrel extension, it will remove all play, and even if the front face is way out of true, the barrel will not be able to move. A much better solution as it does not compromise the anodizing.

[mtdawg169]

Wait, so you're telling me that my senior year trig class actually has a real world application? Get outta here!

Sent from my SM-G920V using Tapatalk

[markm]

I find it hard to believe that a brand with a good reputation like BCM, etc would sell upper receivers that far out of spec.

I think Grant or someone noted that almost every standard upper is untrue to a certain degree. Almost all of my BRAVO uppers are really close to mechanical zero. But this Bushy I'm going to work on is like 12 clicks off.

My tool will be here Saturday. If it looks like the lapping is favoring one side of the upper, I'll shoot it sunday and have quantifiable results.

[Falar]

I hate when this subject comes up. Makes me wonder how evenly my lugs are wearing.

[Joe Mamma]

I think Grant or someone noted that almost every standard upper is untrue to a certain degree.

My personal experience is that there is a lot of variability with the standard AR forged upper receivers. As just one example. I have seen a very noticeable difference in how the barrel extension fit in 2 stripped uppers from a very high quality brand, purchased from the same place, at the same time.

I think the reason more people do not notice the differences is simply because most people do not do side to side comparisons and they do not do multiple builds with all the same parts . . . so they can see the final products are often different.

Joe Mamma

[C4IGrant]

I think Grant or someone noted that almost every standard upper is untrue to a certain degree. Almost all of my BRAVO uppers are really close to mechanical zero. But this Bushy I'm going to work on is like 12 clicks off.

My tool will be here Saturday. If it looks like the lapping is favoring one side of the upper, I'll shoot it sunday and have quantifiable results.

We have not checked every receiver out there, nor do we check every receiver when it leaves our shop. From what we have seen though, BCM, VLTOR and Colt uppers receivers Picatinny rail are square and true. Our guess is that Noveske and Mega Arms (billet and forged receivers) are too.


C4

[lysander]

I think Grant or someone noted that almost every standard upper is untrue to a certain degree. Almost all of my BRAVO uppers are really close to mechanical zero. But this Bushy I'm going to work on is like 12 clicks off.

My tool will be here Saturday. If it looks like the lapping is favoring one side of the upper, I'll shoot it sunday and have quantifiable results.

An out of perpendicular face does not mean a noticeable off-angle of the barrel.

If you re-read the post I showed that an out-of-true condition of 0.004", fairly large, equates to a correction of 3 clicks of windage.

If that 12 clicks is due to the face being untrue, put your barrel in a clam-shell, put the barrel all the way in the upper without tightening down the barrel nut move the muzzle from side to side.

If your front sight moves a 1/4 inch of more, side-to-side, then you, in fact have a bad upper*. If, as I suspect, it does not, then lapping is not doing anything.

[tom12.7]

There's more variables to look at yet, at this point we as a group are not looking at them all.
Maybe we can look into what portions of the bore and face of an upper under the barrel nut torque tension can induce?
Maybe we can look into some of the deviations that occur?

[bp7178]

An out of perpendicular face does not mean a noticeable off-angle of the barrel.

If you re-read the post I showed that an out-of-true condition of 0.004", fairly large, equates to a correction of 3 clicks of windage.

Have you tested this or just come up with a hypothesis? Also, in your 0.004" example which results in 3 clicks of windage; what length sight radius, what is the distance the rifle was zeroed at, and what is the click value?

It would be interesting for someone who had the resources, to take a upper and purposely make it out of square and test. Its a pretty common complaint whereas users will need what they consider an excessive amount of windage. I don't think I've ever read anything definitive about it, just a lot of internet bickering.