system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5:Bullet sizes

what are the demensions? when someone says 55.6 by 44, is that the tip or the casing?
gpt
Caliber is the width of the barrel of a firearm, which should also essentially be the diameter of the bullet.  You'll see two different naming conventions, depending on where/when a round was invented.  

Rounds invented under the metric system will have the diameter in millimeters, followed by the length of the cartridge case.  For example, 5.56x45 has a 5.56mm wide diameter and a 45mm long case.  The length of the cartridge matters because a longer case holds more powder, which means the round will travel faster.

Rounds invented under imperial (standard) units will name the diameter in hundreths of an inch.  For example, .50 BMG is half an inch wide.  Standard rounds do not list the length of the cartridge.  However, most standard-unit rounds have some sort of extra name attached to them to help identify the specific round (.22 "long rifle," .38 "special," .223 "Remington," etc.)