Talk:List of equipment used by the enemy/@comment-208.59.136.36-20140810192008/@comment-175.142.196.93-20141220042234

up to 20mm was still considered machine gun by Japan (mainly because they lack effective classifications), but i've not heard anything larger like 30mm autocannon classified as machine guns.

and the term 'gun' encompass ALL of them, ie: cannons, autocannons, chain gun, gatling gun, rifle, pistol, shotgun, artillery, and YES... Naval Gun as well.

However.. this is moot point because the one OP was talking wasn't Japan's hardware, it was essentially the US hardware, and in pretty much every other military EXCEPT Japan, 20mm is classified as autocannon, not machine gun.

note: part of the reason 20mm are considered autocannons by most military are because this is typically the starting point in which you can create shells with sufficient payload to be effective...

ie: anything smaller than 20mm and putting explosive payload for it is inefficient in general, thus 20mm projectille are for all intent and purpose one of the smallest effective explosive shell.

this is distinct from machine guns, where their caliber are generally too small, that putting explosive payload to their bullet doesn't provide sufficient damage effect to be worth the penalty to their ballistic profile.