I just backed off to check the terminology in question here. Yes, Artbrooks is correct in that the military refers only to heavy crew-operated guns as "artillery". However, some tank rounds are larger caliber than some "artillery" rounds.

Giat Industries provide Tank, Artillery and Medium Calibre Ammunition. These include armour piercing rounds, high explosive rounds and multi purpose rounds. ... 120mm, 105mm and 90mm Tank Ammunition, 155mm and 105mm Artillery Ammunition and Medium Calibre Ammunition ... ...www.armytechnology.com/contractors/ammunition/giat

So is there really any point to making the distinction?