Thread: A Note on what ES and SD are

I think a single digit SD is something reasonable to aim for, from memory, the last time I chronographed handloads I had a SD of 6 fps for a 5 round string, which I was pretty happy with. Assuming muzzle velocity variation can be plotted with a normal distribution (a pretty reasonable assumption) then if a SD of 6 fps is accurate then we would expect 64% of shots to fall between 2744 and 2756 fps (2750 being the average), 95% of shots between 2738 and 2762 fps and 99% between 2732 and 2768 fps; one, two, and three SD's respectively. However If I'm zeroed at 2000m for ammo at 2750 fps and one shot is actually 2732 and the next 2768 then one is going to land 32 inches below POI and the other 32 above POI. So even though 6 fps is a fairly small SD, all it tells me is that I can expect I am going to hit within 32" of my POI, 99% of the time, and within 21" 95% of the time, if my aim is perfect, at 2k. So even with perfect aim and perfect wind reading I can still expect 1 in every 20 rounds (5%) to be at least 21" high or low of POI. Hopefully my maths is correct. I thought one SD was 64% not 68%.