Since 20mm is the cutoff point where most nations switch from bullets to shells, it has come to also generally be the cutoff point between weapons classified as a machine gun or a cannon. This can vary, however: for example, in WWII, Japan classified any weapon over 11mm as a cannon; thus, in Japanese records, their 12.7mm Ho-103 aircraft gun is classified as an autocannon, as it used explosive shells to overcome its lower muzzle velocity compared to American 12.7mm/.50BMG weapons). A machine gun does not fire shells, so historically a weapon that fires shells below 20mm can be a cannon. A true definition will always mention whether the weapon fires bullets or shells in addition to its caliber.