- Of these two measurements, extreme spread and standard deviation, the latter is by far the most important. It's also a measurement not fully understood by many people, thus not often used by non-professionals. This is unfortunate because it's the single most meaningful measurement of velocity results. The mathematical equation used to determine standard deviation is extremely complex, and prior to the existence of computers was seldom used by shooters. Fortunately, these days many chronographs, including the CED Millennium, will calculate standard deviation for you at the push of a button.

Standard deviation tells you how close you can expect the velocity of any shot you fire to be to the average velocity. Statistically, it's been proven that 99.7 percent of rounds fired will fall within three standard deviations above or below the average velocity. 95.4 percent will fall within two standard deviations above or below. And 68 percent will fall within one standard deviation above or below. For example, if our 20 shots generate an average velocity of 1,000 fps with a standard deviation of 10, we can bet the farm that almost every round of that ammo we fire out of that gun (99.7 percent) will fall between 970 and 1,030 fps. Similarly, 95.4 percent will be between 980 and 1,020 fps. And 68 percent will be between 990 and 1,010 fps. Now that's some meaningful data.

A small standard deviation is always a sign of ammunition that's been assembled with skill and attention to detail. By the same token, a wide standard deviation can usually be taken as an indictment of quality control. The more consistent the ammunition, the better the odds it will produce the exact same result, both in accuracy and terminally, every time.

As has been mentioned in this forum for the 450 BM, SD's appear to be fairly large and difficult to get lower, regardless of how much care we take in reloading.

The whole article is at http://www.gunreports.com/special_reports/ammo/Gun-Tests-Ammunition-Testing-Winchester-Cor-Bon-Remington-Black-Hills1895-1.html?ET=gunreports:e1295:138145a:&st=email.