Rifles are rifled and guns are not.
It's that simple?
Tim
I concur, with the caveat that a rifle is sometimes called a gun today; the word “gun” today being used the same as “firearm”.
However, a smoothbore of smoothbore styling should not be called a rifle. I guess in some circles “rifle” is used to mean “long gun”, but that is, to me, incorrect usage. Equally dubious is calling a longrifle a “musket”.