Everybody be careful about which SD is being considered at a given time. There's velocity SD and group size SD.
So how many groups to get an accurate group SD? I may be wrong, but for me to get a fairly accurate velocity SD is 30-50 shots. So for group SD I would think to be fairly accurate minimum of 10 groups, preferably twice that. Again most won’t go to those lengths.
30+ groups.
Your intuition is about right. Starting from a baseline of 3 5-shot groups giving you your long term precision within +/- 25%, going to 12 groups gets you to +/- 12.5%, and 48 groups gets you to +/- 6.25%. Quadruple the number of samples to halve the error. It gets out of hand in a hurry.
As you say, most people aren't going to go to those lengths.
