This is a question that has always bothered me and I've not known the "correct" way to figure it, anybody know? My question arises out of the desire to track a point of impact shift, if there is any taking place. I have a tendancy to believe it's impossible to do with just 3-5 round groups and 10 rounds is necessary? I'm almost positive this is based on the SD of the group but not completely sure. Anybody have a quick formula they use to find the group center which doesn't move the whole center way out because of an apparent flier or two. So far I have just measured the vertical center between the two farthest shots and the horizontal center between the two farthest shots. At the point where these intersect I measure and record the amount it was vertical and horizontal from the aim point and record the scope settings it was shot with. Must the vertical for each shot be measured and SD be calculated for it and then the same for the horizontal to get a more accurate group center? Thanks for any help.