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Purely Academic, but I find it enjoyable

Discussion in 'Ammunition & Reloading' started by Horses are delicious, Feb 20, 2013.

  1. Horses are delicious

    Horses are delicious Willamette Valley Active Member

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    I wrote this for another purpose but thought it might be enjoyed here...

    Materials Selection for Long Range Rifle Performance
    In this essay I will look at factors for selecting optimum components for ballistic performance as applied to long distance shooting. Distances over 500 yards will be considered “long range”. I will address powder, primers, cases and projectiles as well as barrel length and rate of twist. Temperature will be considered but altitude and barometric pressure will not.
    It is my belief that the most important factors to consider when developing a handload for enhanced distances are as follows: short cut, slow burning powders; true, weight separated cases and projectiles with a high ballistic co-efficiency (B.C.) and a secant ogive.
    Aaron Davidson of Gunwerks helps us with our general understanding of what we should hope to accomplish. He states,
    First, let's define a few terms that we need to understand. Muzzle Velocity (M.V.) is just that, the velocity of the bullet at the muzzle of the gun (usually measured in feet per second, fps, and referenced at 15 feet from the muzzle). Extreme Spread (E.S.) is a statistical term that basically defines the difference between the highest and lowest numbers of a group. For example, 35 fps would be the extreme spread for a velocity string of 3000, 3010, 2990, and 3025. Standard Deviation (S.D.) is also used to discuss velocity variations. It implies the probability of dispersion from your aim point. For that reason, it is meaningless for hunters looking to quantify performance. Ethically we need to reduce the chance of an errant shot to zero. That's why I believe that monitoring extreme spreads will guarantee we aren't surprised by a high or low flier. (Gunwerks)
    Selecting the right powder can be a tedious trial and error task involving multiple components and hundreds of loads, or we might luck into the right combination on our first try. There are a small number of factors that can help to isolate some powders as a better place to start your search. Among these is temperature stability. Extreme hot or cold temperatures can have a significant impact on M.V. The idea is to have the same M.V. and point of impact in August as you would have in January. Powders listed as “Extreme” are generally accepted as being well suited to these environments. In this simple test, take five loads in a cooler out to the range on a hot summer day and record the average velocity of five hot shots and five cold ones. That will give you an indication of how that powder behaves at different temperatures. Other temperature factors that could affect velocity include bore dimension changes, and primer effectiveness for extreme cold weather (Davidson). Any match-grade primer should be sufficient but multiple types should be tried to achieve consistent M.V and point of impact. Hodgdon has a line of Short Cut Extreme powders that appear to produce acceptable results for the long-range reloader (hodgdon.com/extreme).
    Next let’s take a look at case selection. Starting with new brass would be advised. You will want to fire-form the brass to match your rifle. This process involves loading the case mildly and firing the loaded cases in the particular rifle you will use for this load, causing the case to expand to fit the exact diameter of your bore. Fire-forming will also take out any imperfections in the case wall, shoulder or neck. Weighing the cases should add another layer of consistency but this may be a fallacy. According to author and reloader Jerry Teo, case volume is a better measure of capacity and therefore accuracy than case weight (Teo). Simply filling the cases with a super-fine medium and comparing difference in volume may be a simpler, more consistent measure of case-to-case quality in a lot.
    Concerning projectiles, their Ballistic Coefficient (B.C.) is the way we quantify the effect that air has on an object in flight. B.C. is one of the best measures for expressing their behavior in-flight and determining a consistent point of impact. The higher a B.C., the less negative acceleration or in simpler terms, the further, faster and flatter it flies. Determining the B.C. and choosing an appropriate bullet for long range shooting will reduce negative effects from long distance bullet drop and wind drift as well as help the shooter maintain a higher level of retained energy at target impact. All commercially available reloading components come with B.C.s listed for the knowledge and advantage of the handloader. Another consideration in bullet choice involves the projectiles ogive, be it secant or tangential.

    Important aspects of a secant ogive are its enhanced ability to overcome resistance to the air and to enhance overall flight stability by having an increased bearing surface while in the rifle barrel. Increased contact with the rifling will produce a truer initial trajectory and a boat-tail design will reduce drag on the projectile and reduce “wobble” in flight (USDC/NTIS). Keep in mind- the desired results are consistent, repeatable bullet placement. These qualities will be increased significantly in projectiles that have both a secant ogive and a boat-tail design.
    A rifles barrel length and rate of twist are other factors which will impact flight characteristics. The rate of twist represents the number of barrel inches it takes for the interior rifling to complete one rotation. The “faster” the twist, the shorter the length required to make the complete rotation. The transverse is true for a “slower” twist. Within the individual ranges of available bullet weights per caliber, lighter weight bullets perform better with a faster twist rate and heaver bullets perform better with a slower twist rate. Exceptions are very rare (Chuckhawks).Barrel length appears to have been optimized by the common manufacturers through extensive research which points to a common set of stock materials which can be made to fit the tooling of multiple common chamberings or calibers. Increasing length to ensure full efficiency or complete powder burn upon the projectile exiting the barrel has led to longer, more accurate shots but even this idea has its detractors (White).

    In conclusion, the value of trial and error in powder selection seems to be more important than choosing powders based on grain size and cut alone. Although presumed that longer cuts will have a slower rate of burn than short cuts, this thinking does not take projectile weight and barrel length into consideration. Generally speaking, light bullets enjoy a fast burning powder and heavy bullets enjoy a slow burning powder. Factory barrel lengths should produce acceptable but not superior results. After some analysis, the weighing of individual cases to determine lot-to-lot consistency has lost some of its merit. It would appear that case capacity is more greatly impactful on load determination than the weight of the individual cases. The selection of projectiles, however, has proven to follow the initial hypothesis. A secant ogive, boat-tail design and a maximum bearing surface are key components to repeatable results in long range shooting.