AR-15 Barrel Harmonics

An approach that can cut your group size by about 20%.

by Joe Carlos

Physics Nobel Laureate Richard Feynman once said, “If you think you understand quantum mechanics, you probably don’t understand quantum mechanics.” I believe most shooters, handloaders, and gunsmiths could substitute “barrel harmonics” for “quantum mechanics” here for an accurate assessment. Although I had to take physics classes in college, barrel harmonics weren’t discussed. I won’t arm wrestle a physicist pointing out some of the points in this article aren’t due to barrel harmonics, but I do know they will significantly tighten groups.

The downside of these techniques is a time and range firing effort to establish correct settings, angles, and pressures. These are not simple “drop in” solutions and many won’t have the time for testing. Some of these modifications require gunsmithing and this will help maximum accuracy tuning.
For decades, shooters have tailored handloads for optimal accuracy with different powders, charge weights, bullets, and bullet seating depth. I perform a load work up for every rifle I build or rebarrel and the customer receives a box of that ammo which will yield good results if properly duplicated. The ammo approach doesn’t improve harmonics much, it’s tailoring ammo to perform with the gun as is. Not everyone can tune ammo. Factory-only ammo and requirements of magazine-fed rifles allow less variables.

American Rifleman published an article about varying the torque on the pillar screws of a pillar bedded rifle to tune for better accuracy. Although I primarily work on Stoner platform rifles lacking conventional stocks and bedding, I sought a similar approach. Torque can be varied on the barrel nut. Begin by properly stabilizing the barrel extension in the upper receiver. Note, Stoner designed the gas tube to pass through one of the barrel nut’s “teeth” and too much or too little torque on the nut results in a bent gas tube, causing the bolt carrier key to misalign.

Read more in the October 2016 issue.

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