Shotgun barrels have been .410 bore, 20 and 12 gauge. Rifle barrels have been a dizzying array of rimfire and centerfire rounds. 
Constant is a configuration consisting of a break-open over-under, configured with the rifle barrel above, and the shotgun barrel below. A manually cocked external single hammer can fire, via various selector mechanisms, either barrel.
 Extractors, and apparently never ejectors, have aided the removal of fired rounds. 
Variation exists in barrel latch mechanisms, shape of receiver at rear, stock materiel, and many other facets of the gun. 
  Most of us on here feel nothing is quite so versatile as a budget priced combination gun. 

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