Friday, December 11, 2020

Brownells AR15 Upper Receiver Lapping Tool - It Works

Brownells AR15 Upper Receiver Lapping Tool - It Works

Back many years ago a number of very high end manufacturers of receivers and match barrels had recommended lapping upper receiver faces to optimize accuracy. Some of those folks were JP Rifles, White Oak, Shilen, Lilja, Black Hole, and Hart just to name a few of the experts delivering that advice. At that point I was younger, less wise, an inexperienced builder and was happy to just screw together one AR after another. 

This advice came back to me as I was chasing down some accuracy issues with an AR15 which should have been a tack driver - instead my first couple rounds were essentially touching and then the groups would string and wander.

Having eliminated all the usual culprits of barrel nut tension, bad crown, scope mounting, I fired up a browser and picked up a Brownells AR15 Upper Receiver Lapping Tool and a tub of Garnett Lapping compound. After using to tool to lap the face of the receiver I was shocked that the $50 investment allowed the premium barrel to deliver tight ½-MOA groups which I knew it was capable of.

The concept of the Brownells Receiver Lapping Tool is pretty simple. The lapping rod is sized to slip all the way into the A15 upper receiver and is then heavily lubricated - thus providing a perfectly straight alignment which the lapping face of the tool can square up the front face of the receiver.  A little lapping compound is applied to the lapping face and the tool is spun with a simple electric hand drill, or in my case I mounted it into my drill press. The lapping tool spins and the lapping compound gently cuts and trues up the face. Usually less than a few thousands of an inch are actually removed in the lapping process. 

The process is a two-three minutes process where new fresh lapping compound is added to the face and fresh oil is added to the spindle inside the receiver. In most situations receivers only needed a little lapping to remove anodizing and true up the face. I did have a couple receivers which did require much more lapping, but that is a story a little later in the article.
Notice how the 4-o'clock to 11-o'clock
position is nice and shiny, but the
11-o'clock to 4-o'clock position
is still black. Keep lapping until
the entire face is silver. Less than a few
thousands of an inch are actually
removed in the lapping process. 

Honestly the whole idea seemed like voodoo to me initially. I thought - Why would it make that big of a difference if the barrel is held mechanically solid by a properly torqued barrel nut? I circled back with those same experts to understand what was going on and why this tuning fix worked. The answer was simple physics and the problem is usually to do with unpredictable and often uneven anodizing and/or receiver coatings.

If the barrel extension is not perfectly squarely seated against the receiver face there will be a small gap on one side or the other. The barrel will expand as it heats and then start to wander usually toward that gap as the barrel torque begins to change. What the shooter sees is 2-3 good shots and then shots wandering off in a direction it is a good indication that receiver truing might be the solution. Another symptom is to only see their best groups while the barrel is hot. White Oak noted receiver lapping is critical, as did others, and even a well trained gunsmith. Everyone noted that a barrel will become less and less affected by barrel nut torque the more true the receiver face is.

Sure it could be something else as well. Too loose or tight of torque, a out of round barrel, bad crown, or out of spec headspacing. JP Rifles noted another culprit is too loose a fit between the barrel extension and receiver - they noted when seating the barrel, it should not drop in and wiggle around in there. JP Rifles actually sizes the barrel and receiver so tight that they need to sweat all the barrels into their receivers for the best possible receiver/barrel fit.
Mounting in a drill press allows for
less side to side lapping tool and
receiver movement and thus a better
final result.

I was able to secure a group buy on 17.3” AR15 .223 chambered barrels based on Feddersen blanks. After already owning two of these barrels and several Feddersen AR15 pistol barrels, all of them have been insanely accurate ½-MOA capable barrel. Actually I have even seen ¼-MOA after long0term break in. Even the 7.5” Pistol Barrels were delivering 1” 100-yard groups - Accurate I tell ya. One of the friends had put together a build with one of the Feddersen barrels. Though an extremely capable and accomplished long-range precision shooter, he was at best seeing extremely varying 1.25” - 1.75” groups with a serious problem of the groups stringing. I offered to take a look and true up the face and assure the barrel nut was properly torqued.

The face of the “on sale $39 budget upper receiver” was extremely out of true. Where the lapping process usually only take 2-3 minutes, I spent over 10 minutes lapping down the receiver. I had never seen one this bad before so it was a great test. After cleaning and reassembling the upper for him, the friend retested and was now producing consistent ½” or better groups from the same setup and ammo. That my friends is a result and the reason I will never assemble another AR15 without first truing up the receiver face. I am now tearing down ever AR15 I have and using the Brownells AR15 Upper Receiver Lapping Tool. I have yet to see a build which the lapping process did not improve the consistency and overall group sizes to some degree.

If you are into AR15 building and into building a quality AR15 at home, then you should consider the Brownells AR15 Upper Receiver Lapping Tool along with tools like a good vise, punches and Geissele Action Rod. The tool has proven to me to be a critical tool to assuring accuracy on a high quality build.

Brownells AR15 Upper Receiver Lapping Tool - $34.99
Brownells Garnett Lapping Compound - $19.99


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Unknown said...

Would this work on the Aero Precision enhanced uppers (M4E1)?


Major Pandemic said...

It will not work on "integrated/monolithic" style receiver, but the good news is that Aero Precision are super precise out of the box.