US Model 1816 "Type III", Percussion Conversion (aka Model 1816 "Belgian" Alteration"); originally .69 caliber flintlock muskets, these were converted to percussion in the early 1850s prior to the grumblings of an impending Civil War.
This is called the "Belgian" Alteration or "cone-type" which is simply a percussion nipple screwed into a tapped hole at the top of the barrel, just off-set to the lock-plate side; the region where the old flintlock flash-pan was is now filled-in with brass. There were two other commonly accepted types referred to as the "French" style - which uses a drum and nipple arrangement, screwed into the location of the primer-hole of the old flintlock ignition system; and the "Bolster" type - which uses an odd-shape "bolster" brazed or screwed into the location of the primer-hole of the old flintlock ignition system.
These utilized the original lock-plates, minus the flash-pan etc. This is called a Model 1816 "Type III" as it has the 3rd and final production changes for the Model 1816; the most notable feature is the enlarged "ball" where the sling-swivel attaches to the front of the trigger-guard. Type IIIs were originally manufactured from about 1831 to about 1844.
Muzzle-loader; black-powder; .69 caliber percussion.
Used by the US at the outbreak of the Civil War, presumed by second-string troops. Also used by the various State Guards or Militias.