Oh, a Smith.
This odd gun was in the company of many other odd guns. It was during the transition from muzzle-loading weapons to breech-loading weapons, when many many breech-loading systems were tried. In Europe this started somewhat earlier, in the 1840's, but in the US ( though there were a few notable experiments earlier in the century, like the Hall) the great change came during the Civil War. A plethora of designs were supplied, by inventors and gun companies, to the Union Army. There were falling-block designs like the Sharps, side-hinged designs like the Joslyn, lever-action hinged-block designs like the Peabody. There were guns operated by levers, by twisting, by breaking open, by operating bolts, flipping toggles, working knuckle-joints, turning cams. All of them shared a central problem: how to seal the breech end, so the gas didn't leak out and hurt the shooter. There were also a plethora of solutions to this problem. The Sharps had a knife-edged ring seal in the breechblock that would flex under pressure against the base of the barrel to seal it, for example. But many would use something like the self-contained brass cartridge case that's used today to hold powder, primer and bullet and seal the breech of a gun- the lever action Spencer, for example. The Smith used a rubber cartridge casing that held powder and bullet but that did not contain the primer: it had a hole at the back end, to allow the fire from a standard percussion cap to ignite it. Many, if not most, breechloaders went to cavalry, where muzzleloaders were the most hard to handle. There were only about 30,000 Smith carbines issued to them , as opposed to around 90,000 Spencers and 60,000 Sharps, and there were problems with all of them, but perhaps the Smith had more than some others. Powder fouling ( gunk left after the black powder explodes) and any dirt tended to make the gun hard to open for loading, and when loaded, hard to latch closed- and either it latched or the gun did not work. The cartridges went into a blind hole, and if the gun got hot and dirty, spent shells could be hard to extract from that blind hole. Also, like the Spencer, the powder charge was rather small, so the trajectory was very curved and the range quite short. If rubber seems an odd thing to have in a gun, it should be noted that the French bolt-action Chassepot ( 1866) would use a rubber gasket quite effectively. But the brass shell with integral powder, bullet and primer pretty quickly displaced all other seal methods.

After the Civil War, the War Department set itself the task of choosing which breech-loading system would be adopted as the standard arm. Although many candidates were submitted by contractors, tested and approved of, including the Smith, the winner was a cheap in-house retro-fit. An armorer at the Federal Springfield Armory came up with hinged-block conversion that could be done on the many many muzzle-loading rifles that the Army now had on its hands, the "Allin Conversion". Once a scandal was swept under the carpet ( one of the other candidates was of the same design as the Allin, and , embarrassingly, already had a patent) this very imperfect solution was slightly improved to become the familiar single-shot blackpowder Trapdoor Springfield, which the US would make in huge numbers and persist in using into the 1890's, long after the rest of the world had gone to magazine-fed smokeless-powder weapons.

Fuller: The Breech Loader in the Service