Eli5 : What are the dimensions mentioned in ammunitions? And how are they different from each other and what makes each one of them unique?

Question

In most movies and video games I have observed people mentioning ammo type and capacity such as, 5.56, 7.76, 9mm, 0.50 calibrate, .45 ACP.

What are these ammo type ?

Edit1: 0.50 Calibre, my mistake!

In: Engineering

Answer 1

Also the higher the caliber the greater the penetration power. For example a .50 cal round will go through an engine block, while a 5.56 probably won’t go through a human body. The different calibers of round are for different things, different targets. You might not want your bullet to go through a person because it’d cause less damage, therefore you’d go with the 5.56 or the like.