“When checking ammunition to ensure it makes power factor (pf), a number derived based on the bullet’s weight and velocity, the most important measurement, of course, is the average velocity for a multi-shot string. Arguably the next most, and still critically, important number would be the standard deviation (SD).
SD is a measurement of of how close we can expect the velocity of any bullet fired, of that load, from that gun, to be to any other bullet fired, of that load, from that gun. According to the author of the Competitive Edge Dynamics Millennium chronograph owners manual, it has been statistically proven (how, I wonder) that 68% of all things measured will fall between one standard deviation above or below the average, 95.4% will fall between two standard deviations above or below the average, and 99.7% will fall between three standard deviations above or below the average.
As a simple example, let’s say we fire a 20-shot string over the chronograph (five shots is generally accepted as the minimum to get any meaningful data, and the more shots the better; I do 20-shot chrono strings) and our average velocity is 1,000 feet per second (fps). Our SD is 10 fps. That means that 68% of bullets, of that load, fired from that gun will fall between 990 and 1,100 fps; 95.4% will fall between 980 and 1,200 fps; and 99.7% will fall between 970 and 1,300 fps.
This can be important information to have. When going to a major match – state championship or above – invariably one of the stages will chronograph every shooters’ ammo to ensure it makes pf. In IDPA they fire three rounds, at least two of them must make pf, or you shoot for no score. In USPSA they fire three rounds and average them; if you don’t make pf, after that there’s a fairly complex procedure you can go through, they want to give you every chance to pass, but no one wants to have to go through that. In ICORE it’s one shot, you make it or you shoot for no score. If all you’ve paid attention to is your load’s average, and that it made pf, but it has a really sloppy SD, it’s possible you might not make pf at the match chrono stage.
The mathematical equation to determine SD is quite complex, and prior to the existence of easily-available, affordable, electronic chronographs, was rarely used by shooters. Today, we can know our ammo’s SD with the push of a button.
This is one area where handloaders have a HUGE advantage over folks running factory ammunition. With factory ammo, you get what you get, and that can include a sloppy SD. Rolling my own, I have produced ammunition with SDs down to 5 or 6 fps. That’s pretty darn consistent.
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