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

Those numbers are the caliber, which is a measure of the diameter of the gun barrel and the ammunition that fits it. The larger the number, the larger the bullet, which generally makes it more powerful. But keep in mind that calibers that are less than 1 (e.g. .50 and .45) are measured in inches instead of millimeters, so .50 is equivalent to about 12.7mm.