Sir, I expect you must be familiar with John Salmon ("RIP") Ford's collected memoirs "RIP" Ford's Texas...
http://www.utexas.edu/utpress/excerpts/exforri2.html
Nary a mention of the '42 rifle-musket in there, but several pertaining the Mississippi Rifle during the 1850's when he was actively patrolling the Frontier, to the point that this was the primary combat arm of his Rangers when their revolvers were "unserviceable" (from what I gather, revolvers were in short supply during the early 1850's).
Mississipi's figure prominently again in his account of his 1860 expedition up into the Indian Territories against the Comanches, to the point that it was Caddos (his own account, others sources have them being Tonkawas) armed with Mississippis what brung down the Comanche Iron Jacket in his old Spanish Armor.
These would, of course be the original .54 cal round ball version.
Never owned but I've handled a rifled '42 at least. A long, heavy and unwieldy thing it seemed, with a conspicuous bright-finished barrel. Didn't help its impact on Texas history any that these things were apparently issued to those unhappy infantry units tasked with protecting the vast Texas frontier against Horse Indians.
Given my druthers, if'n I was tolling around the Texas Frontier in the 1850's, I'd take a Mississippi or ordinary civilian Plains-style rifle any day over a '42.
I expect your hero would too unless he had compelling reasons to the contrary. And Mississippis were apparently around in sufficient quantities such that even Indian allies were issued them.
One has to ask too, how much had minie technology spread in civilian hands even AFTER the Civil War, let alone in the 1850's. Weren't our very first adoption of it even in military hands a Jeff Davis brainchild, in the mid 1850's?
I expect most Americans would first encounter minie technology during the Civil War.
Birdwatcher