Shotguns are described as having a “Bore” or “Gauge” which relates to the size of cartridge that is used.
The historical method of classifying the internal bore of the shotgun is relatively simple. This method involves 1lb of lead being divided into X equal sized spheres,the diameter of these spheres is then equal to the barrel internal diameter and the gauge referred to as X. For example, 1lb of lead is divided into 12 spheres. Each sphere should have a diameter of 0.725 – 0.727 inches. This then equates to a 12 bore barrel.
Thus:
10 bore barrel internal diameter is 0.775 – 0.780 inch (10 balls of lead fit in barrel)
12 bore barrel internal diameter is 0.725 – 0.727 inch (12 balls of lead fit in barrel )
16 bore barrel internal diameter is 0.665 – 0.670 inch (16 balls of lead fit in barrel)
20 bore barrel internal diameter is 0.615 – 0.617 inch (20 balls of lead fit in barrel)
28 bore barrel internal diameter is 0.545 – 0.550 inch (28 balls of lead fit in barrel)
.410 bore barrel internal diameter is .410 inch (odd one out)
There are variations of internal barrel diameter measurement from manufacturer to manufacturer. Individual barrel manufacturers will have a specific diameter for their barrels.
