What is the diameter of O buckshot?

Answer 1

O buckshot is 0.32" (8.1mm) in diameter.

http://en.wikipedia.org/wiki/Shotgun_shell#Buckshot

9 of these pellets weighs an ounce, so there are 144 per pound. A pound is 7000 grains, which means a single O buckshot pellet weighs 48.6 grains. This is just more than half what a typical pistol bullet of that caliber weighs, and less than a third of what a typical 8mm rifle bullet of that same caliber weighs.