Cciafox950 - Your most excellent photos have helped a lot. This is not a Rifle, proper terminology is to call it a Cavalry Carbine. Your photos show this is a Gilbert Smith Patent breech-loading Carbine, and made under contract by the American Machine Works. Although the number on this example is 103, the American Machine Works Smith Carbines were the last ones delivered for use late in the American Civil War. Many thousands of the American Machine Works Smith Carbines ended up sitting in Government warehouses and were never issued issued. That is why so many of the American Machine Works Smith Carbines are still found in unfired condition, of which I'm sure yours is one of them. There was a sporting goods business by the name of "Klein's" (in NYC, IIRC) who was selling these unfired American Machine Works Smith Carbines, still in the dried grease, in 1920 for $2.50, which included an unopened box of original cardboard/brass foil cartridges! Yours is a premier example of an unfired Smith Carbine. The Smith patent Carbines which were issued early in the ACW, and saw service throughout the entire war were made by the Massachusetts Arms Company, and are marked "Mass. Arms Co." on the receiver.
J.