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.

Measure the X and y coordinates of the holes. Put these numbers into a spreadsheet and pick out the ones that you want. Take an average of the numbers in question, Whalla, the center.

I use the same method that 4mesh mentioned except that I shoot on graph paper with 1/4" grid so measuring isn't required for each hole. Just count lines, add up the numbers and divide by the number of shots to get an average. Just make sure that you mount the graph paper with the lines running pretty close to vertical or you can be off a little.