I will venture a definition for 'shotgun': A long gun designed specifically to shoot multiple shot [such as birdshot; as opposed to a single rd ball]. This would include most old style fowling guns but exclude military muskets, for example. By the late 18th cent the British among others had perfected the shotgun and the double-barrel shotgun. Such things as early fusils de chasse and trade guns are shady areas where both shot and single ball loads were common [archeology and old shipping lists show large volumes of both 'shot' [various sizes] and rd ball being sent and used in trade guns, etc. in the colonies. Clearly any smoothbore can fire shot [so can a rifle, but not as successfully]. But such things as smoothrifles are clearly designed for double duty. Military muskets were often used with buckshot. But to me at least a shotgun is a gun designed for small game and bird hunting. That led to lighter, better balanced, better 'fit' [to the shooter] shotguns as opposed to many early 'fowlers', etc...