Designed for the Spencer rifle and carbine, patented 6 March 1860, it was employed by cavalry during the American Civil War, first appearing at Sharpsburg in rifle form. No Spencer carbines were on issue at the Battle of Gettysburg, though two units under Custer had the rifles.
The nomenclature of Spencer cartridges were unique. 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. The first 56 is the diameter of the case at the base .56 inches (14.2 mm), measured just past the rim, and the second 56 is the diameter at the case mouth, also 0.56 inches (14 mm). Later versions of the cartridge included the .56-52, .56-50, and .56-46, which had varying degrees of taper in the cases, to accommodate smaller diameter bullets. All of these cartridges are rimfire primed. The actual bullet diameter of the .56-56 varied between .54 and .555 inches (13.7–14.1 mm), depending on ammunition manufacturer. The .56-52, made by Spencer, and the .56-50, made by Springfield, differed only in the degree of crimp, with the .56-50 having a greater crimp; both fired 350 grain .512-inch (13.0 mm) bullets. The .56-46 fired a 320 to 330 grain .465-inch (11.8 mm) bullet.
.56-56 Spencer Cartridge
Type: Rifle
Place of origin: USA
Designer: Christopher Spencer
Specifications:
Case type: Rimfire straight
Bullet diameter: .550 in (14.0 mm)
Neck diameter: .560 in (14.2 mm)
Shoulder diameter: .560 in (14.2 mm)
Base diameter: .560 in (14.2 mm)
Rim diameter: .645 in (16.4 mm)
Case length: .875 in (22.2 mm)
Overall length: 1.545 in (39.2 mm)
(The above information is from Wikipedia)