Ballistics breakdown
If your customers are competition rifle shooters looking to improve accuracy, then it’s worth understanding harmonics. Believed by many to be the key variable in the development of rifle ammunition, it can make the difference for a podium finish. Here, Paul Bradley deep dives the little known topic and helps retailers increase their understanding when selling ammunition
What are “harmonics”?
Let’s firstly discuss what most people think harmonics refers to in terms of rifle shooting. The commonly shared theory is that during firing a series of harmonic waves travel along a rifle’s barrel. The barrel will therefore be moving as the projectile leaves the muzzle. Both the amount of movement and the timing of this movement are thought to have a significant affect on the projectile’s point of impact. Shooters will demonstrate this by loading ammunition with incremental amounts of propellant and shooting a group from each different charge weight. They will then point to some groups which display a greater degree of precision and define those as “nodes” or charge weights which produce a favourable harmonic. The test itself has much merit but the theory behind it is fatally flawed.
The harmonic does not simply travel up and down a barrel, they pass through the entire system. The rifle, bipod, optic, shooter and even the ground will affect the vibration in terms of both intensity and pattern. If we think of the barrel as a tuning fork (a commonly shared metaphor among shooters), then we must imagine what happens to said fork in the same context. Ring the fork stuck in wet mud vs standing on concrete. Push your shoulder onto the fork while it rings with a heavy winter coat vs a light shirt. In short, the harmonic encompasses far more than the barrel even in the brief time it takes a projectile to travel down the bore. The entire system may even bounce from its position in larger calibres which negates any lack of precision which may have been introduced by the vibratory pattern of the barrel.
In essence every time you change shooting surface or support the harmonic will be affected. Even wearing different clothing may produce a noticeable change if one were to analyse the vibratory patterns.
So why does the charge weight test work?
Shooting groups or ladder tests with incremental charge weights do yield useful results to a reloader but the largest influence on these results is rarely harmonic in nature. You will generally find that an “accuracy node” occurs because an ideal case fill ration has occurred. If you chronograph all your charge weight testing, you will generally see the most precise groups are also those which have very stable velocity spreads. Velocity plateaus when the propellant in the case has reached a volume which allows it to burn in the most consistent fashion. In the simplest term this is usually when the propellant has little room to shift around but has not become so compressed it retains dense areas. When ideal case fill is achieved one can add or subtract a small amount of propellant with little affect on pressure and velocity. This builds a buffer for any small errors you make when dosing a full quantity of rounds.
It is also worth noting that bullet seating depth also plays a significant role in case fill ratio. Many shooters believe that seating depth is only about bullet jump to the lands (rifling). Bullet jump does matter of course but one must not forget that as you seat a bullet further out you gain void in the case. As you seat a bullet further down you may compress propellant further. These factors may have more influence than that of the jump itself. It is easier for us in the commercial world as we are bound to maximum cartridge overall length by both regulation and interchangeability. We mostly build to max length and then worry about case fill ratio. A civilian shooter can choose to run over the max length (chasing the lands as it is commonly known), sometimes by a significant amount. This requires some thought on his part as each change affects the other.
Another significant affect is that of the hot gasses on the projectile as it leaves the muzzle. As charge weights move up in pressure the gasses may begin to bleed around the projectile in the bore and slightly destabilize it on exit. There is a balancing act between case fill ratio, stability upon exit and the benefits of higher velocity. This is what most shooters see when they run a charge weight and call it “harmonics.”
So, do harmonics even matter?
Yes and no. If you are a competition shooter who wears the same clothes and shoots off the same surface every time, then harmonics may be worth some concern. Because they are standardising the variables they can make minor changes to charge weight to glean small precision benefits, and I do mean small. But in competition any advantage helps. For anyone else they are affecting the harmonic with so many variables it would be somewhat of a waste of time. Of course, they should run the charge weight testing but understand what you are tuning, it probably isn’t harmonics. If anyone doubts it, firstly know that many ammunition manufacturers do not even consider barrel harmonics and understand the importance of case fill ratio. Secondly, I would urge you to try a barrel tuner and then shoot the rifle with your “tuned” barrel in a variety of styles and ground types as a hunter or PRS shooter might. You will find the POI and precision changes in the same manner a tuner or a charge weight would affect them.
Rest assured you can build sub half minute ammunition (given the rifle is capable) by simply finding your ideal case fill ratio.
