A local club uses the target, 25 bull, with 1/4" 10 ring, don't recall the org. They basically use NRA rifle silhouette specs for sporter and target class (=any scope, non tapered barrel), and the sporter class has 6x limit. They have an unlimited category for guns over 11 lbs but nobody shoots it. I like the format.The point, not only one, about a max 6X scope is to challenge the shooters. But, in the process many will get discouraged and quit. So, raise the X limit.
Other orgs have half inch 10 rings with x count, or a 100 point center ring with worst edge scoring. All targets at 50y.