Article #8: Barrel Harmonics

Ian

Well-Known Member
#1
The Basement Articles #8: Barrel and action harmonics


Something well known by competitive shooters, but less realized by the average cast bullet shooter trying to improve his groups a little bit, is the effect of barrel harmonics on group dispersion. My general observation has been that a cast bullet is more sensitive to these things than jacketed bullets, and that taking them into consideration when developing a load can have some very real benefits even for a person not bent on achieving sub-MOA groups.


To sing well, your rifle must sing in tune, and the primer and powder gives the voice. Let me attempt to describe the basic scenario of harmonics. The trigger is squeezed and the firing pin is released, propelled by spring pressure until it strikes the primer. As it strikes the primer, the case is driven forward in the chamber if there is any room for movement, and is "loaded" with pressure against what ever is stopping it, be it the case shoulder, case rim, or bullet touching the rifling or throat. As the priming compound detonates, it begins a uni-directional shockwave that deflects from the cup and is focused forward through the flash hole. This heated pressure wave encounters the powder in the case and it begins to ignite and build pressure in all directions, forcing the case to expand to the limits of the chamber and breech face. There may be a jolt as the case stretches and the head slams into the breech face. By the time the bullet, being the intended "failure point" in the system, begins to move, the case is "loaded" under pressure in all directions. As the bullet begins to move, the pressure wave develops a forward direction and the case reacts in the opposite direction, causing the beginnings of recoil as it loads against the breech face and through the action, bedding, stock, and finally against the shooter's shoulder. In a way, all this loading and preloading is like winding a spring due to microscopic flexing within the system, and it begins to unwind as the bullet is slung forward through the barrel. The barrel acts like a buggy whip as the launch stresses bow the chamber down and away from the muzzle due to the vectors created by the action support points and the static inertia of the muzzle end of the barrel.


The bullet is literally surfing a wave as it passes through the barrel.


The wave might overtake the bullet, remain behind it, or coincide with it depending on the timing of the pressure build. When the bullet exits the muzzle, the muzzle is in a state of "whip", either up or down. Obviously, if the bullets fired in series exit at slightly different times in this muzzle movement, the group will exhibit some stringing. Sometimes it's vertical, sometimes horizontal, it just depends on how the vibrations are affecting the muzzle.


While the bullet is riding the primary wave in the barrel, many other small things can affect it, such as the higher-frequency "traveling wave" that may cycle between muzzle and breech several times before the bullet leaves the crown, and bedding points or inertial points such as barrel bands and gas blocks tend to break the primary wave into separate "nodes", or first or second-order harmonic vibrations between the points. A barrel is like a guitar string: Play it "open" and it vibrates between the bridge and the nut, the bedding points. Fret it in the middle and it plays the first-order harmonic an octave higher, on both sides of the fret point. Fret it anywhere else and it will vibrate at two different frequencies, sometimes complimentary, sometimes not. If you "chime" a guitar string by lightly touching it at a first, second, or third-order harmonic point (like the fifth fret or in the middle) when plucked, the string will vibrate at a much higher frequency than open, even though the node creating the high note was only applied long enough to set up the short-wave vibrations. A rifle barrel is much the same sort of system, and all of these vibrations can affect accuracy by changing muzzle position slightly each shot.


We cannot eliminate these vibrations, but we can minimize the "noisy" ones and do our best to refine the others into a predictable pattern that we can control so our singing and dancing launch platform always puts on the same act every show.


So how do we get these vibrations under control? Fortunately, the rifle manufacturers did most of the hard work already by engineering (usually, but not always!) fairly intelligent bedding points and by not hanging too much junk on the barrel. Fixing loose or weak bedding, isolating the barrel from the stock, and making sure things stay tight is a good start. Most of the remainder of the work lies, believe it or not, at the loading bench.


Let's re-examine the description of the firing cycle, with harmonics in mind. All that case movement and expansion sets up little "rat-a-tat" noises throughout the gun, as does the firing pin flailing around. A strong firing pin spring, snug-fitting cartridge case, and well-lapped bolt lugs quiet this down, especially as the full pressure of the firing event swells the chamber and puts a shockwave through the entire action. Minimizing slack and movement of the parts makes the whole action move as a unit instead of like a string of boxcars banging one into the next against the slack in their couplers. Consistent primer ignition is crucial to being able to repeat the same harmonics each shot, so primer seating pressure and finding a primer your "system" likes is important. Consistent powder ignition and pressure build is also crucial, which is why selecting a powder that burns correctly for the gun (or using tricks to MAKE it burn correctly such as fillers, buffers, and duplexing) and tuning the charge carefully is so important to accuracy.


Something else often overlooked by reloaders is the importance of having consistent case neck tension gripping the bullet. If the tension is inconsistent between cartridges, it will release the bullet at different points in the pressure curve of the burning powder and begin to change the volume of the space available to burn. As the volume changes, so does the burn rate again. All this affects the relative position of harmonic waves with relation to the bullet, and changes the timing of muzzle exit, and ultimately causes deviations in muzzle velocity and which point on the target the bullet is "flung". Crimp also affects bullet release to a certain degree, and can hold the bullet a few fractions of a second longer to let pressure build slightly more before letting go of the bullet just as a drag racer loads his engine against the transmission brake and releases it for launch. Seating a cast bullet hard into the lands or letting it jump a bit before engaging the resistance of the rifling also have a huge effect on harmonics and pressure curve of the powder.Exactly what is happening is less important than being able to load ammo that does whatever it does just the same way every time, so very often there is no "best" technique for things such as how far out to seat a bullet. The gun will tell you what it likes if you experiment with the variables.
 

Ian

Well-Known Member
#2
(Continued).....

Once you get your system tuned up and mechanically sound with good bedding, a good hold, and well-fitting ammunition, it's time to start on load development, and I will address powder selection and such in the next article, but for now I will assume a suitable powder for the intended performance range has been selected and is being tuned.


Your targets will tell a story if you learn how to read them. Observe patterns of dispersion throughout a load workup, firing minimum five-shot groups. Ten-shot groups are much better, although trends can be seen in as little as three shots if you know what you are seeing is harmonics and not the effects of poor loading techniques, lube, bad castings, or rifle anomalies. Huge, evenly dispersed, shotgun patterns are typically caused by powder that is far out of tune with the gun. If you try several different charge weights and the groups don't start to form some sort of linear string or close up, try a different powder because the rifle is telling you it isn't going to shoot with that, no matter what the loading books or your own rationale might indicate. Here's a trick someone shared with me that changed the way I worked up loads forever: Look for the first sign of vertical stringing in a group because it's telling you that you're getting close. That's just the way the vibrations tend to all add up. Bump the powder charge just a little more if you have room, and watch the string close up into a round group. If you've done everything else right and the system is capable of it, the group will close into a hole. Adding more powder will open it up again, sometimes making a visible horizontal string. This pattern can repeat several times in a workup, only once, or not at all depending on how the powder is interacting with the rest of the system. Once you get the powder selected and tuned as well as you can, start adjusting things like seating depth, neck tension, primer brand, crimp, bullet sized diameter, bullet weight, and lube to see if groups can be further improved. With each adjustment, the powder charge may need to be fine-tuned again to achieve the best results, because everything else affects powder burn rate slightly.


There have been many lengthy volumes written on load development, theories and techniques of load workups, and even barrel harmonics and external ballistics, so it is impossible to cover it all here, but hopefully I scratched the surface of the theory and practice of perfecting cast bullet groups through techniques that consistently isolate ideal vibrations.


Here's a link that may be of interest to those with internal ballistics software, Chris Long's "traveling wave theory" as discussed on RSI, interesting stuff that WORKS: http://www.shootingsoftware.com/barrel.htm


Ian