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

5.56, 7.62. Etc are all measurements of the bullet width. If you have a box of ammo it’ll say, for example, 7.62 X 45, that is saying the bullet width is 7.62 mm and the case length (the brass that holds the gunpowder) is .45 mm long