With the increase in interest in long-range shooting, the terms extreme spread (ES) and standard deviation (SD) are being thrown around a lot. What exactly is ES and SD? More importantly, should you really care about them? And what do they mean to the expected performance of your ammunition? I will offer some definitions and examples for the first question and the answer to the second question depends on whether you are a handgunner or rifleman, a plinker or a precision shooter.
First off, let’s discuss the definition of ES and SD. ES and SD are mathematical terms that define the extremes, uniformity and expected variation in a sample of data or numbers.
ES is pretty simple, it is the extremes of values of a set of data or numbers. As it applies to ammunition, it almost always refers to either the velocities or pressures produced by a sample of ammunition. Let’s say we test 20 rounds of ammunition and measured the pressure and velocity of each round. The ES of pressure would be the difference between the highest and lowest pressure. Likewise, the ES of velocity would be the difference between the highest and lowest velocity. As it turns out, with ammunition, the pressure and velocity ES go hand in hand. What one does the other will mirror. You’re going to get small pressure ES with a small velocity ES and vice versa. The more consistent the components of the ammunition are and the more consistently they are loaded the lower the ES you would expect.
SD is not as straight forward a concept. To be technically correct and to satisfy all the math majors out there, let’s start off with the mathematical definition of SD. The SD of a random variable, statistical population,