That looks like a rifle-musket, three barrel bands/40 inch barrel-- a proper 1855 rifle would have a 32 inch barrel and only two bands. When Stonewall Jackson looted and burned the Harpers Ferry Armory, a lot of rifles were assembled from the parts at the original Palmetto Armory, a lot of them with fragmentary Maynard locks (the back plate with the high hump and the high-arched hammer) replacing the tape primers, which were never popular with troops or anybody else, with nipples for regulation caps. There was also a musketoon with one band and a 24 inch barrel, but I have only seen pictures of them in the Armory documentation from before the War. I've been trying to get the parts for an 1855 lock together to build a rifle on for quite a while now, being a damn poor has not helped-- (In the interest of full disclosure, I may or may not have once been Sgt in a Louisiana Zouave reenactor unit)
ETA: Should be bored for .58 Minie balls, will feel about like shooting slugs in a 20 gauge shotgun