Christopher Spencer wanted to get his new gun adopted by the Army during the American Civil War. But he wanted to bypass the beaurocracy of the Army Ordinance Board. So he went directly to President Lincoln. He gifted the President with a carbine, and they tested it right on the White House lawn. Lincoln liked the gun so much that he basically ordered the Army to buy the guns.
The Spencer is a 7 shot carbine that feeds from the butt stock in a tubular magazine. It was used at the end of the war, and subsequently through the Indian Wars until it was replaced by the Trapdoor Springfield.
In the 1870's, there were thousands of these guns in storage in warehouses. So in 1871, Springfield Armory received a shipment of 1,100 of these carbines for refurbishment. The inspector got permission to convert them into rifles that were more suited for the infantry. This is an unusual variation of the Spencer.
The caliber is 56-50 rimfire. The cartridge has seen a revival due to the popularity of cowboy action shooting, but is available only as a centerfire version. Fortunately, Buffalo Arms makes a replacement block that allows you to use the centerfire cartridge.
It only takes a short period of time to install the drop in conversion block and it doesn't require any alterations to the gun.
The rifle is now ready to fire modern cases. I highly recommend you load your centerfire cases with the original black powder charges, as these guns are well over 100+ years old. Modern smokeless loads are not recommended in such an old gun. Chiappa makes replicas of modern, stronger steels. If you decide to fire smokeless loads you might be better off choosing a modern replica.