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ACCURACY VELOCITY

By Frank Haertlein
Former President CVC.
( California Varmint Callers Assoc.)

 

Being an avid varminter and hunter, I am always interested in obtaining the best practical rifle accuracy possible. Over the years I have collected various forms of data in regard to rifle accuracy and my intent here is to share some of this information. Hopefully, the reader may be able to derive some benefit from this experience. To some of you this may just be re-inventing the wheel but none the less it serves to expand on one aspect of the accuracy equation.

Jim Struble (former president of the CVC .) and myself conducted one rather lengthy experiment with a Remington 40-X in .22-250 and a Remington Model 700 in .243 Win. Both rifles had heavy barrels and the usual accuracy enhancements of pillar bedding, trigger work, etc. We ran these test in order to find the most accurate possible loading for both rifles. As any experienced reloader knows, the number of variables that enter into the search for accuracy are almost infinite. With this in mind we selected the best components then available while attempting to keep as many variables out of the equation as possible. The same brand of case and primer were used throughout the test. No changes were made in the sizing, cleaning and case preparation techniques. Some variables such as boat-tail, flat base, hollow point or spire point were not considered. All bullets would be seated to just touch the rifling. The only allowed variables were bullet weight, bullet manufacturer, charge weight and powder type. After expending literally thousands of rounds, we were able to get both rifles shooting to competition bench rest standards. We kept records of the brand of primer, powder, bullet weights, cases, seating depth, sizing techniques, velocity, group size, etc, etc.

All of the tests were conducted at 100yds under ideal conditions. Velocities ranged from a low of 2800 FPS to a high of 3800 FPS for the .22-250. Five rounds were fired for each loading. Any extreme flyers were removed from the data and attributed to human error or other unknown variables. The net effect was simply to smooth the graphed data. In either case the data trends could be clearly seen. Four months after the shooting stopped we had what we thought were a number of good loads for both rifles. I will attempt here to limit this discussion to the .22-250, although the same set of circumstances existed with the .243 Win. Just the numbers were different. Analysis of the data revealed a very interesting and useful phenomena that we had not been expecting. This became apparent as we plotted group size Vs velocity with different bullet weights and powder types. All of the loadings in a given caliber were plotted on the same graph. From the graph it can be seen that as velocity was increased from a 2800Fps arbitrary starting velocity the group sizes for all loadings became smaller until a point of group size convergence was reached (convergence as defined here is where the group size becomes smallest). As velocity was further increased from this point there was again an increase in group size but ultimately to a lesser degree of dispersion than that seen at the lower velocities. Figure 1 illustrates this behavior.

Accuracy Chart

Figure 1. Various loadings

Extrapolating the data trends past the 3800 FPS maximum of the test indicates another convergence or decrease in group size would occur somewhere around 4100 FPS. Bullets ranged in weight from 52 Gr. on up to 63 Gr. What we found most interesting was, regardless of bullet weight, profile, manufacturer or powder type, the group size went to minimum around a specific velocity. All of the different bullet weights showed best accuracy around 3600 FPS with the 26" heavy barrel of the Remington 40-X. (about 3300Fps for the .243 Win.)

Taking into consideration the .22-250 and the .243 test results, the data indicated that such factors as bore size, barrel weight ,length, and contour determined the velocity where best accuracy is obtained with a given rifle. Different bullet brands, weights and powder types had an affect on accuracy but to a smaller degree than did velocity. Given an appropriate selection of quality components, velocity was the single biggest determinant of good accuracy. For example, if the rifle shot 2" groups at a arbitrary starting velocity of 2800 Fps with a given powder and bullet combination, the group size shrunk to 1/2" by increasing the velocity to the "accuracy velocity" of the barrel. Further reductions in group size could be realized by selecting different powders, bullet brands and weights while keeping the velocity at or near the "accuracy velocity" of the rifle being tested. With what would later prove to be an optimum selection of components, the best groups were not achieved until arriving at the correct "accuracy velocity" of the rifle.

The best grouping rounds with the .22-250 (36.9 Gr. of H380, 52 Gr. Match bullets, Fed case, Fed 210 BR primers and 3600Fps velocity) were shot in a number of different but similarly constructed rifles. The rifles shot less than 1 inch groups and with minor adjustments in powder charge were going into 1/2" or better. The same situation existed with the .243 and we were able to try this with a greater number of different rifles in this caliber. Going further, the experiments showed that with a shorter barrel length, the best "accuracy velocity" increased. With a 24" heavy barrel for the .22-250 we found that a velocity of around 3700 Fps was necessary for best accuracy. Certainly, other points of convergence could have been found at velocities below the 2800Fps minimum of the test, but this was not done as it would have been defeating the purpose of achieving excellent accuracy at what we considered usable velocities.

From an analysis of the data it can be seen that every rifle barrel will have a natural frequency of vibration defined by it's length, weight, contour, bore size, the type of steel it is made from, etc. A shorter barrel is stiffer than a longer barrel and thus has a higher frequency of vibration. Imagine, if you will, a set of wind chimes and how each chime has a different tone or frequency of vibration because of it's length. The explosion of powder in a case is somewhat like hitting the end of the barrel with a hammer. The barrel will vibrate or "ring" at it's natural frequency. Any unwanted harmonic frequencies or damping, such as when wood stocks or other accessories come into contact with the barrel, can have unpredictable results by causing the bullet to be released into flight at random points in the barrel's frequency cycle. Accuracy suffers as a result. A free floated barrel will usually give the best consistency and a specific velocity gives the best accuracy. Whatever weight of bullet you use that is reasonable given the twist rate of your barrel and as long as you can speed up that bullet to the barrel's "accuracy velocity", you will generally get good results.

As demonstrated by the 24" barrel of the .22-250, the shorter barrel will provide for a point of convergence at a higher velocity than the longer barrel. But a longer barrel has more velocity potential and this could be enough to bring into use the next higher point of convergence. Exactly where the convergence points occur in the longer barrel will of course depend on it's construction. The shorter barrel could be denied the use of the next convergence level as it may not be able to drive the bullet fast enough to reach it before bumping into the pressure limits of the rifle.

With all this in mind it may be possible to predict the "accuracy velocity" of a given barrel with actual measurements of it's vibration frequency. You could strap an accelerometer on the outside end of the barrel and then correlate the vibration frequency to accuracy. This may be an oversimplification of the process but nonetheless I think it could be done. Empirical data of any correlation between what I call "barrel factor" (length, weight, contour, caliber, material, etc.), frequency of vibration and accuracy could be developed. Conceivably, this could allow a barrel or rifle manufacturer to select a particular barrel contour and length thus producing a barrel suited to a particular application. I have not heard or read about anyone attempting this and I would be interested in hearing from anyone who has. Undoubtedly, if you wanted to get the maximum velocity and accuracy out of your cartridge there is some combination of barrel length and contour where a point of convergence could be made to fall near the higher, velocity limit of the bore size in question. Unfortunately, unless your a gunsmith, this is not practical. Of course there are other considerations in this equation. Barrel life, case life, efficiency, strength considerations, cost, etc. Developing a load that shoots in your rifle is a time consuming and labor intensive effort and, hopefully, armed with this information we can develop loads with a more informed approach. But there is another approach to the accuracy problem.

In reading the latest issue of VERY HIGH POWER (VHP 1996 #2 -- The .50caliber shooters association quarterly publication) I am beginning to hear rumblings about barrel vibration and muzzle brakes. I see this as a good trend and in fact have wanted to experiment in this regard for a number of years. Unfortunately, my job has taken me to Australia. Because of my work, I have been unable to do much shooting let alone experimentation with adjustable muzzle brakes or barrel vibration. I do not attempt here to be a proponent for the new line of Browning rifles, but merely use them as an example of a major manufacturers attempt at overcoming the accuracy problem. The Browning BOSS (ballistic optimization shooting system) is a method of altering the vibration characteristics of a barrel. What you have, in affect, is a fore and aft adjustable barrel weight with a built in muzzle brake. By adjusting the weight outward you are lengthening the barrel and decreasing it's vibration frequency, vice-versa when you adjust the weight inward. What you are doing is tuning the rifle to the load as opposed to tuning the load to the rifle. This gives you the ability to choose a load that provides the desired velocity and then adjust your rifle to shoot best at that velocity. Browning has a patent on their BOSS but this need not stop others from the development of an accuracy enhancing system. One possible approach is to keep our existing muzzle brakes but add an adjustable weight to the barrel. In any case, adjustable barrel weights provide the shooter with a whole new approach to reloading for accuracy and promises to make load development a snap. In a pinch, you can even get your rifle to shoot factory ammo with acceptable accuracy.

Good Hunting!
 

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