Tim Singleton said:NBRSA rule book pg 26
(13) GROUP MEASUREMENTS. Groups are to be measured by any method approved by the NBRSA in .001 inch. The Sweany Type Reticle Rule (or its equivalent) will be the only official measuring device used at all Registered Matches. In measuring groups fired with calibers larger than .22 (unless the Reticle is calibrated for the caliber to be measured) the measurement shall be made from the extreme outside edges of the 2 widest bullet holes and the actual differential of the larger calibers shall be subtracted from the measurement read on the measuring scale. (See: Target Drawing in back of Rulebook.)
Targets are to be marked in the upper right-hand margin, indicating reduction in measurement (6-mm reduction of .019 and 308 reduction
I believe the Niel Jones tool is the most commonly used at matches and is the same design as the Sweany tool which is no longer made.
http://www.neiljones.com/html/target_measuring.html
That is how group measuring is done at registered NBRSA matches
(unless the Reticle is calibrated for the caliber to be measured)