Most, maybe all, chronos these days calculate standard deviation (SD) for you, but other than knowing that (generally) a small SD is better than a large SD, very few handloaders know what use can be made of that information. Here's one practical application for hunters or anyone else who's interested in seeing how a particular range of velocities translates into vertical stringing due to the difference in velocity alone.
If the distribution of your velocities is "normal" in a statistical sense, i.e., follows a bell-shaped curve (and all of the velocity data that I've worked with is pretty close to normal), then for all of the possible times you shoot that load/rifle combination, about two-thirds of the velocities will be within +/- 1 SD of the mean, 95% will be within +/- 2 SD, and 99% will be within +/-3 SD. Knowing that, you can take your mean (average) velocity, subtract 2 SDs, add 2 SDs, and know that 95% of the time (probably good enough for most of us, but use 3 SD if you like) the velocity of your next shot - or your first shot at game - will be within that range. Knowing that, you can take any of the commonly available trajectory tables/calculators and plug those numbers in to see if the resulting vertical spread is going to be acceptable for the game and expected distances you'll be hunting. If so, you're done, if not it's back to the reloading bench, or perhaps back to the LGS to try some other factory load.
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NRA Certified Pistol Instructor
Pemigewasset Valley Fish & Game Club
Last edited by Unclenick; December 8, 2012 at 05:13 PM.
Reason: fixed math typo