There are many brands of varnishes out there that use tung oil. Spar varnishes for marine use are typically (and have been for a very long time) tung oil based, and they are extremely tough and weather resistant. The gimmickery employed by some manufacturers is the, "WOW, ours is a tung oil finish."
I much prefer the pure tung oil, maybe a bit harder to work with, but to me the result is well worth it. I love the fact that perhaps once a year (or if finish gets scuffed or scratched) after hard use I can hand rub in one light coat of tung oil and the wood is restored back into great shape.
Another reason for my preference perhaps, is the tung oil we used in the army a half century ago, it worked well on an M-1 then, and still does for me today.
With all that said, I doubt if a tung oil type finish of any kind was seldom, if ever used on original muzzleloaders in North America, I don't think it was even widely available in the US until the late 1800's.