Two Triggers,
I resurrect lots of old guns. Sometimes they can be rebarreled and the old barrel saved for posterity, sometimes they can be relined. There are a of folks hunting with old Sharps and Rolling Blocks and Ballards in the larger calibers. I use an early 1890 Winchester on squirrels and rabbits if not an 1885 original.
The underhammer is the world's simplest (and perhaps ugliest) action. Used for muzzleloaders, it has a mainspring that is often also the mainspring, and a hammer and trigger - nothing else is required although sometimes a sear spring is used as well. These are muzzleloaders and have direct ignition from the nipple into the bottom of the powder chamber(barrel). They are highly reliable, simple and ugly. There are underhammer flint locks but most are percussion rifles.
Brent