Accuracy and Chronograph Testing, Part 8

posted on October 7, 2016

WARNING: All technical data in this publication, especially for handloading, reflect the limited experience of individuals using specific tools, products, equipment and components under specific conditions and circumstances not necessarily reported in the article and over which the National Rifle Association (NRA) has no control. The data has not otherwise been tested or verified by the NRA. The NRA, its agents, officers and employees accept no responsibility for the results obtained by persons using such data and disclaim all liability for any consequential injuries or damages.

Above: A still-life in ammunition—certain to warm any dedicated handloader’s heart!

In this article, we will conclude our discussion of chronograph testing and evaluating handloads for Service and Match Rifles. Part 7 provided much background information and may be helpful to review for those who missed it.

Modern chronographs usually calculate a wide variety of statistics for each sample, including the velocity Average (mean), Extreme Spread (ES, or the difference between highest/lowest velocities) and Standard Deviation (SD). Standard Deviation is probably the least widely-understood of these.

Essentially, SD is a numerical reflection of how much individual velocities vary around the mean. Higher numbers equal more variation in individual shots, and lower numbers indicate less variation. However, there is a catch: for the SD to be valid, the sample size must be adequately large.

Reaching back several eons to a misguided youth spent studying Statistics, the sample size greatly affects the accuracy of the SD’s prediction. In academic or industrial quality control statistics, sample sizes tend to be rather large—many times more than what is practical for the majority of NRA High Power rifle competitors. A minimum sample of 30 is strongly preferred.

Within the marksman’s practical limits of, say, 20-shot or 10-shot samples, the SD can help one assess and compare the velocity uniformity of different loads. (Obviously, 20 is significantly more predictive than 10.) However, with a 5-shot, or especially a 3-shot sample, the SD generates little hard information; the sample is too small to adequately reflect the load’s population of potential shots to be fired.

Consider this string of 10 velocities from Rifle X, fired from a cold, fouled bore as is frequently done in NRA High Power competition: 2829, 2936, 2788, 2830, 2841, 2803, 2801, 2835, 2837 and 2836. If one sampled only the first three shots, much variation would exist: ES equals 146 FPS, and the average equals 2851 FPS.

If one’s random sample happened across the last 3 shots to be fired, however, the load would look great—on paper! (ES equals 2 FPS!) The average, 2836 FPS, is not far from the first sample’s average, but the superb uniformity demonstrated in this 3-shot sample would be very misleading.

When considering the entire, 10-shot sample, one finds the ES still equals 146 FPS, the SD equals 40, and the 10-shot average equals 2830. Another load with a much lower ES and SD would be preferable, assuming accuracy met the required standard. Extremely uniform velocities alone are *not* a reliable predictor of accuracy!

Standard Deviation: A Useful Tool
Readers wishing to learn more about using SD to evaluate their load’s shot-to-shot consistency might search the internet for a brief tutorial. When used properly, SD can be a valuable tool. For approximately “normal” (bell-shaped) distribution curves, a rule of thumb is that approximately 68 percent of shots will fall within +/- 1 SD of the mean, approximately 95 percent within +/- 2 SD’s, and essentially all will fall within 3 SD’s of the mean.

Thus, if one’s average velocity is 2700 FPS with a (valid) SD of 10, approximately 68 percent of shots should fall between 2690-2710 FPS, and approximately 95 percent within 2680-2720 FPS. A different rule applies to any set of data, even those with a non-standard distribution. That approach (beyond the scope of this discussion), while not as precise, still allows prediction of approximately 75 percent of shots within +/- 2 SD’s of the mean.

Once one has determined that their load is (A), sufficiently accurate at the target distance and (B), that velocity and uniformity meet the desired goals, the chronograph provides still more useful data. It allows the shooter to predict wind deflection (based on the bullets’ Ballistic Coefficient), trajectory and even remaining energy at various distances if desired. As with a rifle/load’s accuracy evaluation, these predictions should be verified by firing at the actual distance, to avoid unexpected surprises.

The further the target distance becomes, the more challenging predicting ballistic effects such as wind deflection and drop (with precision) can become. What is easy at 600 yards becomes noticeably trickier at 1000 yards (+).

As a young man, this writer was one of very few shooters to own his own chronograph (back in the Dark Ages). At that time, having the ability to actually measure one’s velocity, rather than having to guess, was simply revolutionary! Although generations of handloaders did rather well without them, once one has used a chronograph, loading without one would seem primitive and imprecise, at best.

One Final Rule
Don’t shoot your chronograph!! (Yes, really!) The author has personally witnessed gruesome chronograph deaths several times at the hands of even experienced shooters. It is not a sight for the faint of heart.

They simply fell victim to a few common pitfalls. It is very easy to become distracted when shooting over a chronograph for the first few times. Losing track of where the sights are, relative to the muzzle, can be hazardous for your sky-screen’s health.

Remember, there is a significant offset between the muzzle and the sights/crosshairs at short range. When in doubt, err on the high side—i.e., shoot a bit higher than one thinks might be optimum for sensor readings. Firing ammunition without getting a reading is a lot less expensive (and embarrassing!) than replacing sky-screens!

Many chronographs have light-diffusing shields supported about 10” above the sensors by two angled holders in the shape of a large “V.” Especially when using a scope of medium-high magnification, it can be startlingly easy to shoot these supports.

If one’s magnification is high enough and parallax is not set for short range, the supports can be virtually invisible in the field of view. Experienced chronograph users often adjust the scope to its’ lowest magnification and parallax to its’ shortest setting. This makes the supports plainly visible in the field of view, and helps considerably.

Also, pick an appropriate aiming point on the target or backstop. This helps ensure bullets cross the sky-screens along a consistent path and well away from the downrange apparatus. Finally, before firing the first shot, it is helpful to leave the rifle on the rest, aimed at one’s mark on the backstop, and view the bullet’s flight path from the side. Is it too close to the screens? If so, aim higher.

SSUSA thanks the U.S. Army Marksmanship Unit for allowing the reprint of this article.


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