Question: how big is 20 mm round?
Answer: 20 mm cartridges have an outside bullet diameter and inside barrel diameter of 0.787 inches (20.0 mm). Projectiles are typically 75 to 127 mm (3–5 in) long. Cartridge cases are typically 75 to 152 mm (3–6 in) long and most are shells, with an explosive filling and detonating fuze.