Anyone lap the upper receiver on their AR builds?

[Bang]

I see it as a surface that might corrode when the protective anodizing is not there.

[Hodgie]

I see it as a surface that might corrode when the protective anodizing is not there.

I have never seen an aluminum receiver corrode even 80% that are completely left in the white after being built.

[Bang]

6061 and 7075 aluminums have excellent ambient corrosion resistance. However, they are subject to corrosion caused by reaction via contact with dissimilar metal. A steel barrel creates that condition. Not saying it is something to worry about, just saying removing the protective benefit of anodizing to accomplish a questionable result deserves consideration.

[unclesam099]

https://www.thefreelibrary.com/Maxim.....-a0520713898

Great article about accurizing the AR and providing examples of what worked and what didn't.

EDITED - Here is the same author, claiming 14% smaller groups with this fix:
https://www.thefreelibrary.com/Strai.....-a0565200743

[Bang]

From the above reference:

AR gunsmiths can buy barrels with the same quality used by gunsmiths building bolt guns. So if two "identical" barrels come out of a given factory on the same day and one ends up on a gas gun and the other on a bolt action rifle, why does the latter usually shoot tighter groups?

We'll consider receiver squaring first. Gunsmiths building custom bolt guns sometimes buy quality custom actions that arrive squared. When using lower cost receivers, the diligent gunsmith will perform squaring operations to the receiver face (front).

For decades, AR gunsmiths have turned a blind eye to this process. With the exception of some very high-end receivers, like those made by Sun Devil, most AR receivers come from the factory with run out with one side of the receiver face taller than the opposite side. When the AR gunsmith pushes the barrel extension into the receiver, the raised flange on the extension will contact that high side of the receiver face and stop. As the barrel nut is drawn up and exerting pressure, the barrel will be set at an angle inside the receiver. Since there is so much slop in the fit between the barrel extension and the corresponding inside surfaces of the receiver, there is no resistance to the barrel seating at an angle. If the bolt carrier assembly is perfectly aligned with the receiver when it delivers a cartridge to a barrel that is set crooked, there is no way that a cartridge attached to the bolt face is going to line up as well as we want with the center of the barrel's bore.

I believe that runout in the receiver face likely influences barrel harmonics as well. As the barrel whips, those high spots constitute a pivot point making shot-to-shot harmonics less uniform. Those uneven harmonics likely cause the barrel extension to relocate to different angles inside the receiver. The barrel extension becomes a moving target for the bolt to try to align with for each shot. Most gunsmiths assemble the AR rifle and leave all that slop between the barrel extension and the upper receiver, permitting the extension to move around which causes the muzzle to return to a different position after each shot, resulting in fat groups.

The solution is twofold. Start by properly squaring the faces of your receivers. I performed a study on this. I built temporary guns on a bunch of receivers and machine rest tested them with known lots of match ammo. Then I disassembled each gun, properly squared the receivers, and reassembled each temporary gun using the same parts, torque settings, etc. Using the same ammo lots, the squared receivers improved by a significant 14%.



Let's be clear on the squaring procedure. This needs to be done on a lathe. I tried a squaring tool that fits in a hand drill. Using lapping compound, the long section fits into the receiver and it's supposed to result in proper alignment of the squaring surface as even, forward pressure is applied to the drill. However, used as recommended, I measured the run out of a receiver was about half again greater than the undoctored first reading. I chucked that tool in the trash. Comparing once again to bolt gunners, they do their receiver face squaring on a lathe and not by running something in the receiver attached to a hand drill.

With the receiver face properly squared we are part way to a solution. Next is to stabilize the extension in the upper receiver. This is best done by matching a BAT Machine custom barrel extension to the upper receiver to achieve a tight fit. You can also use a thin (0.001") section of shim stock wrapped around the stock extension and held with green Loctite 620. Properly done, this keeps the barrel extension in one consistent position at the front of the receiver and the muzzle of the barrel will return to the same spot for each shot, cutting group size by about 1/3. This is so important that I wrote about it in, "The Relationship Of Barrel Extension Diameter To Accuracy In The AR-15" (March and April 2013.)

[Gunplummer]

Most people in the AR-15 threads seem to buy parts as cheap as possible. Think it really matters? How many do you think LOAD, not reload ammunition for their AR's?

[imashooter2]

Most people in the AR-15 threads seem to buy parts as cheap as possible. Think it really matters? How many do you think LOAD, not reload ammunition for their AR's?

Well there*s me.