Years ago, a number of high-end AR manufacturers recommended lapping upper receiver faces to optimize accuracy. At that point I was a younger, less-wise, and inexperienced builder who was happy to just assemble 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. Each time I fired it at the range, my first couple rounds were touching and then the groups would string and wander.
Having eliminated all the usual suspects of barrel nut tension, bad crown, and scope mounting, I fired up a browser and picked up a Brownells AR15 Upper Receiver Lapping Tool and a tub of Garnet Lapping compound. Two days later, I was shocked that the $50 investment had produced a tight ½-MOA rifle.
The Concept of the Brownells Upper Receiver Lapping Tool
The idea here is pretty simple. The heavily lubricated guide rod is sized to slip all the way into the A15 upper receive, thus providing proper alignment for the lapping face of the tool so it can be used to square up the 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 a drill press.
The lapping tool spins and abrasive lapping compound gently cuts and trues up the face. The process takes two or three minutes, during which fresh lapping compound is added to the face and fresh oil is added to the guide rod spindle inside the receiver. Most receivers only need a little lapping to remove anodizing and true up the face. I did have a couple receivers that required much more lapping, but I’ll talk about those a little later in this article.
Voodoo or Physics?
Honestly the whole idea seemed silly to me initially. I thought, “Why would lapping the face make that big 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 fix works. The answer was simple physics, and the problem usually has to do with unpredictable and often-uneven anodizing and/or receiver coatings.
If the barrel extension is not seated perfectly squarely against the receiver face, there will be a small gap on one side or the other. As the barrel heats up during firing, the metal will expand and any gap can allow the barrel to warp due to the gap. What the shooter sees is 2 or 3 good shots, followed by shots that string away from center or otherwise create overly large groups. Another symptom is to only see tight groups when the barrel is hot.
Proof it Works
I was able to prove the concept when a friend built his own rifle using a Fedderson barrel from a batch that I had used successfully to create 1/2-MOA and even 1/4-MOA rifles. Although he’s a skilled and accurate shooter, his best groups were 1.25″ to 1.75″ at 100 yards, and the groups were stringing badly. I offered to take a look, true up the face, and ensure the barrel nut was properly torqued.
The face of his “bargain basement” upper receiver was extremely out of true. Although the lapping process usually only take 2 or 3 minutes on a typical receiver, I spent more than 10 minutes lapping his to true it up. I had never seen one this bad before so it was a great test.
After cleaning and reassembling the upper for him, he took it back to the range and found that he could produce consistent ½” or better groups using the same setup and ammo as before.
That my friends is a result, and the reason I will never assemble another AR15 without first truing the receiver face. I am now tearing down every AR15 I have and using the Brownells AR15 Upper Receiver Lapping Tool on them. I have yet to see one on which the lapping process did not improve the consistency and overall group sizes to some degree.
If you are into AR15 building or would just like to build a quality AR15 at home, you should consider the Brownells AR15 Upper Receiver Lapping Tool along with other AR tools like a good vise, punches, and Geissele Action Rod. The lapping tool has proven quite valuable in assuring accuracy on any aR build.
- Brownells AR15 Upper Receiver Lapping Tool – $34.95
- Brownells Garnett Lapping Compounds (3 types available) – $19.99