Primer: How To Measure a Group - 05/05/12
Okay, having been justly chastised for incorrectly "correcting" Travis on the other thread, here's what I meant to provide by way of illustration. I've used circles drawn around pushpins to illustrate the centers and the edges of bullet-holes.
The centers of the holes, measured by calipers, are easy to do here because they're not holes. The only way you can accurately measure from center-to-center on REAL bullet-holes is to use a transparent caliber-specific overlay, which will give you a fairly precise center estimate to measure from. But even then there's a lot of room for measurement error, so it's less precise than a caliber-specific obturator, which will give as precise a center-to-center measurement as you can get without a computer and optic.
The best way to measure without using an overlay is usually to measure from the same-side edge of the two holes. Since the edges of real bulletholes are ragged, with a "grease ring" around the hole. Typically we measure edge of the hole rather than the greasering, hence the popularity of semiwadcutter bullets--which cut nice clean circular holes--for Bullseye shooting. So sameside edge to edge measuring is my preferred method:
The alternative method, which Travis suggested, is to measure the opposite-side edges of the two farthest holes ("outside edges", as he calls them):
...and then subtract the bullet diameter from that measurement.
When I was a regular Bullseye shooter, the guy who ran our league (and was an ex-Navy Pistol Team member) alwasy insisted we measure from sameside edge to sameside edge, as that is the way they apparently do it at Camp Perry.
In the end, it really comes down to this:
Do it however you want.
The centers of the holes, measured by calipers, are easy to do here because they're not holes. The only way you can accurately measure from center-to-center on REAL bullet-holes is to use a transparent caliber-specific overlay, which will give you a fairly precise center estimate to measure from. But even then there's a lot of room for measurement error, so it's less precise than a caliber-specific obturator, which will give as precise a center-to-center measurement as you can get without a computer and optic.
The best way to measure without using an overlay is usually to measure from the same-side edge of the two holes. Since the edges of real bulletholes are ragged, with a "grease ring" around the hole. Typically we measure edge of the hole rather than the greasering, hence the popularity of semiwadcutter bullets--which cut nice clean circular holes--for Bullseye shooting. So sameside edge to edge measuring is my preferred method:
The alternative method, which Travis suggested, is to measure the opposite-side edges of the two farthest holes ("outside edges", as he calls them):
...and then subtract the bullet diameter from that measurement.
When I was a regular Bullseye shooter, the guy who ran our league (and was an ex-Navy Pistol Team member) alwasy insisted we measure from sameside edge to sameside edge, as that is the way they apparently do it at Camp Perry.
In the end, it really comes down to this:
Do it however you want.