Gauge (firearms)

The gauge (U.S. English) or bore (British English) is the inside width of the barrel of a firearm. 
So, a 12-bore shotgun could take a cartridge with a solid sphere of lead which fits a 12-guage bore. Actually, the gun fires a cartridge which looses off a cluster of small lead pellets. They in total weigh the same as the single lead sphere which just fits the barrel.
Lead is poisonous, and builds up in the environment. By 1919, the spent lead pellets from waterfowl hunting was identified as a major source of deaths of bottom-feeding waterfowl. So the tendency is to switch to steel pellets in the cartridge. Dense (heavy) plastic pellets in cartridges is another option, standard in competition shooting.