I had some reloaded shotshells tested at HP White labs...They wanted 5 rounds to get a 95% accuracy rate on velocity and pressure...20 rounds gave them a 97 1/2 % accuracy if I remember correctly... They told me 4 rounds they considered minimum to get an average of about 90%... Took a lot of money to have ammo tested then...like 20 bucks per shell. I wanted a real 75 yd goose load. Got one too. Looonnnggg way with a shotgun
Anyway that was the way they figured the statistics