There was a lot of experimentation with shot sizes in late XIX century Britain - finding the 'magic' number of pellets per load which would kill grouse (partridge, pheasant) best. However, from the marketing perspective, naming the new shot sizes according to No of pellets per ounce or per load was confusing for customers, so if the new size was smaller than 5 but bigger than 6 it was called 5 1/2, and so on. 7 1/2 is about the only survivor of the battle.

Russian Hunting Shotguns