It would be vicious. There's historical documentation of a shot made with a .50-90 (1872 Sharps buffalo rifle round) at 1500+ yards, but I'd imagine that took several feats. However, the energy of the round itself doesn't exceed that of a .308, in fact it's in the low-middle range of modern hunting rifle rounds. So I'd give it a stat block like this based on the DMG rules for firearms and a Sharps:
Name Damage Weight Properties
Buffalo Rifle 2d10 Piercing 10 pounds Ammunition (Range 400/1600), Reload (1 Shot), Two Handed
This will make for an extremely long range weapon (the given ranges for a hunting rifle in the DMG are silly, BTW) which fires faster than a musket and can be reloaded using a bonus action, allowing a shot every round without feats. If you let the character split iterative attacks with the reload bonus action that will allow two attacks with it per round from a long frigging way off.
I'd also introduce feats to extend the range of the weapon, primitive optics that allow for long range shooting without disadvantage, and even faster reloading.
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