Here is a 1/4 minute target . . .
Attached is a graphic of the NBRSA/IBS benchrest target for Hunter Class with 6x scopes, and the Varmint-For-Score Class with unlimited scope power. The rings are 1/4 inch apart. The 10 ring is 1/2 inch in diameter. 9 ring is 1 inch. 8 ring 1 1/2 inches, etc.
The target was designed by John Sweeny back on 1968 for Hunting rifles with scopes not to exceed 6 power. John was the man who designed and built the first Reticle Rule for measuring group size to the nearest .001 inch.
The target was designed for a distance of 100 yards. It is used on NBRSA and IBS SCORE matches. One shot at each bull. Five target cards, for a total of 25 shots in a match. No group shooting in these matches. TOUGH because you must move the rifle's aiming point to a different target for each shot. In tournament competition a scoring gauge is required to determine when the leadilng edge of a bullet hole touches a scoring ring.
With the scoring rings 1/4 inch apart it is easy to make sight adjustments with 1/4 minute clicks.
Ahh: This target is used for sanctioned centerfire matches only. The National Associations have different targets for rimfire competitions.
Rimfire E-Mail Benchrest Match Coordinator