In 1855, Benjamin Franklin Joslyn designed his first breech-loading rifle. After successful tests, the U.S. Army ordered 50 of them in .54 caliber, but quickly lost interest in the rifle, as they had far more interest in the burgeoning cavalry carbine development that was sweeping the industry.
In 1861, Joslyn designed a modified version of the rifle in a smaller carbine format. Using a metal rimfire cartridge, the Ordnance Department ordered 860 of these carbines which were delivered in 1862. These are referred to as the M1862 pattern carbines with most going to units from Ohio. In 1862, Joslyn received an order for 20,000 additional carbines, with delivery starting in 1863, but by the time the Civil War came to an end, only about half of these had been delivered.
Joslyn subsequently made several upgrades changes to his design and came up with the M1864 pattern, making several times as many as the M1862. 16,500 were made, with most going to the Union Army. It accepted a standard .56-52 Spencer cartridge and was far more popular with the Army. Interestingly, Springfield Armory subsequently bought approximately 3,000 Joslyn breech assemblies directly from Joslyn and built brand new rifles with them right at the tail end of the war, the first mass produced breech loading rifles made by the Springfield arsenal.
The initial M1862 used metal rimfire cartridges in .56-52 caliber and had a laterally hinged block called the "cap" which enveloped the standing breech and could be swung open to the left when the locking catch was released. This design was refined in 1862-63 with the addition of cam surfaces which improved the cartridge seating and extraction. It was chambered for the .56-56 Spencer rimfire cartridge.
The M1864 featured many small improvements and refinements to the M1862 design, with some even converted to center fire. The M1864 was also modified to accept a proprietary .54 caliber rimfire cartridge made by Joslyn. The patent markings were moved from the top of the breech block to the rear surface of the breech block, and the buttplates were marked with US, while the M1862's were not.