For many people, the underhammers are a great option forbuilding a roundball gun that has the capability of easily switching barrels.
I have an old Numrich Buggy Rifleunderhammerso I'm familiarwith it, and these aren't my cup of tea. The stockdoes nothave as modernof a design,the 2 piecetrigger is not asresponsive, the trigger guard mainspring is crude and the hammer islocated too close to the wrist and just not conducive for easy cocking(while hunting for instance).
Inline actionsbuilt withmodern 1 piece pistol grip stocks are more ergonomic and just look at the adjustable Timney trigger that this inline action has.That's an advantage for any gun to have built into it, evenif this action isn't the mostperfectly designed.
This is just one of those unique products that I happened across that stimulatesa person'simagination into consideringnew muzzle loading riflepossibilities and configurations.
A person can design a riflehavingthe specific length, caliber, twist and even the depth andtypeof rifling of their choosing. Possibly they couldturn it intoa switch barrel rifle to cover different needs and uses, similar to the TC System One with a shotgun barrel, and small & large caliber barrelsavailable for it, but with even more options. With the right action the possibilities are endless really, so people's choices aren't limited to only factory production models and features.

Showing this action was justintended to be food for thought. If money weren't an objectto considerthen I'm willing tobet that more folks would considerbuilding themselvesa semi-custominline to indulgein their hobby, especially if they could affordablybuild oneto the exact spec.'sof their choosing.