Question: what is the difference between 12 bore and 20 bore?
Answer: The 12 refers to a 12th of 1 pound of lead or more specifically a pound of lead can be turned into twelve balls of equal size that equate to the barrel diameter of what we know as a 12-bore (0.729"). This also explains why a 20 bore is smaller - i.e. the pound of lead divided into 20 pieces.