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Number of Shots per Group
I agree with Outpost. Artful's solution sounds like a good one too.
With my competitive shooting background, almost all my load and rifle testing is with 10 shot groups. Some 30 calibre hunting rifles will get several 5 shot groups as a test, but varmint and target rifles are held to a 10 shot group standard.
Revolvers are held to two cylinders worth for a test and semi autos at least one or two full magazines.
Here's a good example:
A Remington 788 in 30/30 was test fired at 200 yards with the classic 311114 bullet. The first three rounds were something like 1 1/4 inches. I stopped there just for fun and after using a different aiming point (top center edge of the target) to separate the two groups, shot the remaining 7 shots of the intended 10 shot group into something like just under 4 inches. The three shot group? Not worth believing while the 7 shot group, very good at 2 MOA, is much more representative of the load's capability at 200 yards.