read the description above the photo where it says converted to rifled.....I'll paste it here....
Breech loading smoothbore percussion carbine, Model 1843 Hall - North, 1850, converted to rifled, .58 cal.
The Hall series represented the earliest military effort to develop a breechloader, these shorter carbines, although made in smoothbore, were percussion primed from the beginning, this was the last in the series, manufactured by Simeon North, Middletown Connecticut, 1843 to 1853. A thumb lever on the right was used to pivot the entire breech above the barrel for loading, with a gas release hole below the breech to vent on firing. Carbine was originally configured as a single shot .52 cal smoothbore breechloader, iron buttplate with flat shoulder, iron trigger guard, thumb lever opens the breechblock, fixed iron sight, one flat band held by pins, iron nose cap serves as second band, saddle bar with riding ring, ramrod with button head and threads is used for cleaning. During August 1861, a number of these carbines, now obsolete as a smoothbore, were converted to .58 caliber rifled, this was done under contract for General Fremont, the contract became the subject of a major scandal due to excessive profiteering by the contractor, referred to as the "Hall Carbine Affair" at the time. Mark on top of frame: "U.S. / S. NORTH / MIDLTN / CONN / 1850". Top of barrel is marked "STEEL" Additional inspector mark "JCB" on barrel. Barrel length 21in.
Ref: Flayderman 9A-278.