Re: I have a question about determining group size.
Very nice shooting! Your other option is to measure the holes center to center. Depending on the kind of paper your targets are made of, the holes will often not be .308" in diameter, but smaller. In that case, measuring outside to outside and subtracting .308 will give you a smaller group size than it actually is. You can easily determine this by using your caliper to measure an isolated hole and see whether or not it's .308" (.3") in diameter. But for most, that method should be more than close enough that it really shouldn't matter.
The down side to measuring center to center is that you have to eyeball the centerpoints as best you can, so that is one possible source of error using that method. IMO, the bottom line is, measure the best you can and if it's off by 1/10", so be it. My aim is by far the limiting factor in my group sizes, not the method I use to measure, so I always round the numbers after measuring to the nearest 1/10" anyway.
