Does my rifle barrel really flex?
Category : Reloading
The path to greatness when it comes to precision rifle reloading is to tune the ammo to your barrel. It’s a well documented fact that when a bullet enters your barrel from the chamber, it sets the barrel into motion. It creates a force wave, in the form of a complex sine wave, which travels the length of your rifle barrel, and upon reaching the end, will bounce back until the wave energy is consumed by the steel molecules. This sine wave action isn’t centered around any single portion of the barrel, but is affected by the details of your barrel.
To get a true sine wave, a few things are necessary. First, the barrel must be installed properly to the receiver. Any error in that connection will cause the sine wave to be dull and clunky. It should, when done right, allow the barrel to ring like a bell. Secondly, the barrel must be free floating. Well, that is the ideal situation. Not every gun is made like that. However, be aware that anything touching the barrel along it’s length (especially the stock) will dampen the sine wave, in a negative way. Finally, an overlooked factor… The received must be fastened to the stock properly.
Custom stocks are often created with a set of pillars in place that the receiver attaches to. The structure pretty much eliminates the variations in 
wood and some plastics due to heat and humidity. Wood expands when wet, and shrinks when dry. Such cycling of a non-pillared stock can cause the screws that attach the receiver to the stock to loosen over time. Every knowledgeable precision shooter knows to check the torgue of those screws with a torque wrench that measures in inch pounds. Wheeler makes one that does the job well for under $40.
So, given that the conditions are correct tor propagating a clear sine wave, how do we use that to tune the ammo to the barrel? Well, first let me demonstrate what the wave form looks like. In class, I use an old M1 Carbine barrel, hanging from my fingers, and tap it with a 45 acp pistol barrel. The clear ringing (sine wave) can be heard. I then draw a two dimensional sine wave on a pad of paper to show that there are nodes along the pathway where the barrel isn’t moving up, down, or sideways. That’s the sweet spot. That’s what you want to adjust the travel of the bullet to, such that the bullet leaves the muzzle just when it is not being pushed upon by the barrel sine wave. I usually use a wooden dowel rod to suggest the flexing shape.
Recently, my son was visiting, and he captured my M1A service rifle in the act of firing, in slow motion. If you slow down that video to it’s slowest speed (click on the cog icon, and select .25 speed), you will clearly see the rifle’s barrel flexing. Had the abilities of the iPhone camera been even more sophisticated, you would see the bullet leave the barrel right in the middle of a flex (point of NULL). Here’s that video, change the play speed to .25 and full screen for best viewing. Watch the very end of the barrel:
If the barrel is flexing upward and to the left when the bullet leaves the muzzle, the bullet (and bullet group) will be pushed that way. Etc. I have seen countless examples of variations of +/- .1 grain of powder move the bullet group from one corner of the target to another. Yes, you can adjust your scope to wherever that group exists. But, suffice it to say that the most accurate groups will be when the barrel is in the NULL position, at the not-moving portion of the sine wave.
In my reloading class, I offer a technique to dispense and measure powder for rifle cartridges in hundredths of a grain vs tenths. And, I explain and show how to adjust the powder load so that you find that sweet spot in the bullets velocity for the best accuracy.
The concept that the fastest bullet flight is the most accurate is a fallacy. The fastest bullet flight may yield the flattest trajectory. But, a properly tuned bullet velocity (usually somewhere between starting load and mid range of velocity) is the place where you will yield the tightest group.
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