The Springfield Model 1861 rifle musket was produced in over 1 million examples during and after the American Civil War (1861-1865) and used by both sides of the conflict. However, its muzzle-loading arrangement made it obsolete in an age when the breech-loading system was growing into standardized usage. This led to some manufacturers converting the famous long gun into breech-loading forms by reworking the original flintlock system in play. One such conversion - undertaken by the Meriden Manufacturing Company of Connecticut - produced the "Miller Model 1861". The gun carried the surname of "conversion block" patent holders William H. and George W. Miller. In the system, a release catch found along the top of the barrel was used to brake the action open along a hinge, exposing the breech for inserting a cartridge. The patented arrangement was also featured in Parker's Snow and Company Model 1861 rifles.
Conversion work on the guns spanned from 1865 to 1867 and, by and large, the weapons retained their general form and function complete with the long single-piece wood body with integrated grip handle and shoulder stock. Two barrel bands were used to secure the barrel to the wood body. Sighting devices were set over the receiver and over the muzzle for ranged fire in the usual way. The trigger and loop guard were underslung beneath the action. Overall length measured 47.2 inches with a barrel of 39 inches. Weight was about 10lbs.
Despite the modification to breech-loading, the guns remained single-shot weapons as the Model 1861s before them. They were chambered for a .58 caliber rimfire cartridge. The reloading process, however, was much improved over the original Springfields as a trained shooter could manage up to six shots per minute. In the original rifle muskets, an operator could hope to reach a rate of fire of about two to three rounds per minute.