Think about it... what does .50 cal mean??? 1/2 inch! What does 30mm mean? 3cm. What is 3cm in inches? 1.2 (ish...) The large (20mm) shell is not more than twice the size of the .50 cal. Ergo 20mm.
why do you have so many stereotypes about canadians lol, we can own guns but you have to have a trip permit to carry them outaside your home, we just cant carry them while were working at our jobs like americans