Has to be a smooth 24" from muzzle to point of ignition and <2" bore to be classed as a shotgun. You can load ball on a home office approved range but your fellow shooters will be diving for cover and banning you PDQ if your gun is choked
Usually choice would go for a ML musket with more metal than your average shotgun at the high pressure end of the bore so you can really stoke it up.
Plenty to choose from, original or repro, flint, percussion or match.