Most 20mm ordinance used in WW2 carried an explosive payload, and weren't simple kinetic/conventional rounds. However, even a conventional 20mm round is quite large. The WW2 20mm ammo was typically 138mm long, which equates to a .79 caliber round 5.5 inches long. That has enough mass to kill someone if struck in the head, falling back to earth at terminal velocity.
Most 20mm ordinance used in WW2 carried an explosive payload, and weren't simple kinetic/conventional rounds. However, even a conventional 20mm round is quite large. The WW2 20mm ammo was typically 138mm long, which equates to a .79 caliber round 5.5 inches long. That has enough mass to kill someone if struck in the head, falling back to earth at terminal velocity.