What is a 30-30 caliber?

Early on in the development of black powder cartridge guns, the calibers were numbered by the bore diameter in inches with the second number being the powder charge weight in grains of black powder. Therefore, .30-30 is a .30 inch (caliber) bullet with a powder charge of 30 grains of black powder. Other calibers of the black powder cartridge era include the following examples: .25-20, .38-40, .44-40, .38-55, .45-70, .45-90, .45-110, etc. It's interesting that some of these cartridge names lived on to the current day, even though they are loaded with smokeless powders. Even the grand old "thutty thutty" as it's known in New England.