Technically true; however, in practice, you must rifle the barrel of any grenade launcher, or else have tail fins on the grenades to prevent aerodynamic tumbling. Tank guns, cannons, howitzers, and the like typically have rifled barrels but we don't call them rifles. (Mortars, on the other hand, use finned ordnance.) AFAIK rifles are usually defined to have a caliber at most 20mm.

Ah, but we do. See for example http://en.wikipedia.org/wiki/Recoilless_rifle .

Most of which, ironically, are smoothbores.

I thought that tank guns these days are usually smoothbore rather than rifled - with the notable exception of the British Challenger 2:

http://en.wikipedia.org/wiki/Challenger_2

The British have been trying to replace that rifled gun with the German L/55 smoothbore, too, but they don't have the money. It wasn't clear at the time the tank was designed, but the rifled barrel turns out to have been a mistake. The round for which it was primarily designed (High Explosive Squash Head, or HESH) hit a sort of technical dead end with the introduction of spall liners and layered composites in tank armor.

Read the comment by Steven Den Beste, who explains the situation pretty well (aside from getting the HESH acronym wrong):

http://www.professorbainbridge.com/professorbainbridgecom/20...

True, but the smooth-bore guns used in most other tanks all fire fin-stabilised rounds.