a 16 ga+14 mm combination gun probably build by Johann Nepomuk Schemmetzler in Bruck an der Mur around 1830 as a muzzle loader , later rebuild into a breech loader for center fire cartridges by an unknown gunmaker. the 14 mm rifle barrel will using 28 ga cases, imho.

https://www.dorotheum.com/de/l/6462251/#