I'll add a thought here on the underhammer subject.
We have a privately owned museum in Arkansas called the Saunders Museum in Berryville.
The museum contains hundreds of muzzle loading pistols. The majority of them are underhammers and some go way back to the beginning of the percussion era. I think Col. Saunders (not the chicken guy) had a preference for pistols and that is why so many are represented. Point is, the action was popular and widely used from the start of the perc. era up to, well....I guess....present time.
A friend of mine worked in the museum as a kid maintaining the guns. He now makes a really nice underhammer rifle he sells.
And, a digression, there is a local range named after Col. Saunders and there are two big ml shoots there every year.