I don't think there's a rule...Just whatever works. I know a guy who shoots long range using his middle finger. Hold's the bolt with the thumb and forefinger and extends the middle to the trigger. Says he can shoot faster (cycle faster). Has big hands... I tried it and it doesn't work for me.

No rules, Just whatever works I think. I'm pretty good with a rifle at long range, ok with the handgun but not as good. I practice both a lot though. Keeps the skills I do have sharp. It's fun too.