Quote:
Originally Posted by catnthehat
Easiest way for me is to measure outside to outside then subtract a calibre width, but at times I just measure outside to outside and be done with it.
My match rifles are designed to shot groups but to shoot scores past 500 meters, however, so I don't do a lot of shooting off the bags at 100 except to work up loads, then quite often I shoot ten shot groups.
Cat
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If you subtract caliber diameter you will likely get a false measure - one actually smaller then the group really is. To be accurate, you need to measure a single hole on your sheet of paper and deduct this number from the group measure. Be sure to measure the same marks on the single hole as you do on the group hole.
If the group is a little larger so you have distinct holes on the two most extreme holes you can measure on the same side of each hole and not have to do any subtractions with a gage hole diameter (from the paper) or an actual caliber diameter which in most cases (if not all) will give you a false measure.