In my .300 Win Mag Ruger M77 MkII Stainless. I measured the group two ways. One, I measured outside to outside as it laid, and subtracted the bore diameter. On the 2nd measurement, I pushed the paper pedals back from the backside, measured OS to OS again, and subtracted the bore. I'm sure I did something wrong that the old fellers will point out, but I'm happy.