When I held matches, I would get the groove diameter of the barrel, measure from outside to outside of the widest shots and subtract the groove diameter from that distance. For score tragets, it's much harder (center line of ring, bullet being in or out). I use my best jugdement on those and ask other shooters to make a judgement, also. The problem there is the ring is curved and it's hard to know where the bullet passes through it, since it's no longer there. I had no device to help with it.

Frank

Hello experts! 

I am also verrrrry interested in learning how you measure these groups in US. We do not have these matches in Germany, so I don't know that - I read only about that.

But  do not understand how it works. Is somebody able to post some pics of that procedure? Who can explain that in pictures?

        Biggi from Bavaria.    

Neil Jones of Custom Products in Pennsylvania supplies this accessory to standard vernier (dial or digital) calipers… It's used at nearly all formal benchrest competitions here in the US. The black arrow points to one of several inscribed bullet diameter rings on a clear plastic "overlay."

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With the tool at zero, a selected ring is placed over a bullet hole. The leftmost component has a rectangular block (with a retractable pin) that's pushed down to hold "base" in place whilst the overlay is adjusted to the right and centered over the most distant bullet hole. This returns a reading to .0001" for the Center-to Center distance between the selected holes.

Other diameter rings can be inscribed, to better fit the larger BP bullet hole. An accessory magnifying lens can be fitted to the slide to more accurately position a ring over the bullet holes, in my 20+ years use I haven't found it necessary.

dwd

P.S. for some of the time, I need the 18 inch caliper.