Redbob said: "I've never understood why these rifles were never issued during the Civil War."
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There's some confusion about that. There's an official government record that indicates the remaining post-war inventory of this pattern gun happens to match the numbers of the 1982/63 Remington contract alone.
The hitch might be that the government didn't specifically consider this a new pattern of gun, but rather a follow-on order of the 1855 Harpers Ferry and prior "Mississippi" 1841 rifle of the same pattern. Each iteration had improvements and noticeable-to-us differences in the hardware, but nothing so radically different that the government would assign them a different rack in inventory. In other words this Remington contract was essentially just a follow-on order of the the same 2-band pattern previously accepted. Because of that, the inventory didn't specify withdrawals of the 1863 iteration specifically, meaning that some of them could have gone out during the war (horrors and screams from the campaigners here).
In any event, this Remington contract rifle, is what today we call the "Zouave" because some Zouave units used rifles of the pattern (2-banders equipped with the long-bladed bayonet).
All said and done the 1862/63 iteration represents the peak of cap-and-ball rifle technology, before breech-loaders supplanted them as the primary issue rifle. These Remingtons were used for worldwide shooting competitions after the war, in part because these fine shooting rifles were made available as surplus.
Because it had such a good reputation in the post-war, the type was among the first the Italian reproduction makers felt would sell, and early reenactors found the repros (or even surviving originals) an attractive buy. As repro 3-banders came into the reenacting mix the two-banders became (unfairly) reviled as inauthentic and not safe in rank fire with three-banders. In truth the 1863 repros were good guns, just misunderstood, even if some repros were better than others.
The one pictured in the OP here seems a gem, wherever it was hiding out. If nothing else a primary example of a CW-era cap and ball rifle, and perhaps still quite viable for live-fire if the bore checks out, though as an artifact it has more value I would think. If a repro, someone has taken great effort to de-farb and apply period markings.