Handloader1, forget standard deviation.....and look at groups, if the groups are good , to hell with the standard deviation, yes groups are associated with SD.....but the SD is not what puts chucks in the ground.

Most of the posters are absolutely correct about the SD not being relevant to diameter.....it's simply a factor of variation and the number of shots fired. Let the Chrony do it for you.....then forget it.....look at the groups....screw the math.....go with the results you can see!!!