No Spencer carbines were on issue at the Battle of Gettysburg, though two units under Custer had the rifles.

It was loaded by a variety of companies, and was also used in the Ballard and Joslyn Model 1861 non-repeating breechloading rifles and carbines.

Unlike later cartridges such as the .44-40 Winchester and .45-70, where the first number indicated caliber and the second the charge weight, the .56-56 refers solely to the case.

Later versions of the cartridge included the .56-52, .56-50,[2] and .56-46, which had varying degrees of taper in the cases, to accommodate smaller diameter bullets.

The actual bullet diameter of the .56-56 varied between .54 and .555 inches (13.7-14.1 mm), depending on ammunition manufacturer.