Yes Thompson Center made one rifle, now discontinued, that seemed to cross the gap between traditional and the modern "muzzle loading" rifles. At a distance it looked like maybe it was lever action, but then there's no lever and a ramrod is seen. Very pretty rifles, and not too "offending" to the eye of a traditionalist (imho) with about the same amount of metal to wood as an H&A Underhammer (which does meet our criteria).
I often thought that they should've made them in .54 and .58 instead of the .50 and .54 versions, as the .58 might've found a niche.
A 24 gauge won't blow up the squirrels unless you're literally right on top of them..., because you have no choke. What you will likely get is a pretty good pattern at 25 yards... if you use the Skychief Load . What it will also do is give you a round ball that will hammer deer, likely out to 50 yards or so.
LD