Well.... I hate to belabor the obvious, but a rifle is a gun.
Hmm. Gee, then I wonder why the British firms that made both referred to themselves as "Gun & Rifle Makers" on their signed work and trade labels. Were Holland & Holland, Purdey, Rigby, Boss, Evans, Atkin Grant & Lang, Gibbs, Churchill, etc., just plain stupid?
If a gun can't have rifling, why do we all know battleships are well armed with guns and not rifles?
Battleships are armed with heavy naval rifles, not guns. The smoothbore muzzle-loaded naval weapons that armed men-of-war of the past are properly referred to as guns.
Sure, the word "gun" is widely misused, but repeating the error endlessly doesn't make it correct. A shotgun is a gun, a rifle is not.