system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How are ammunition sizes chosen?

 

I have always wondered why ammo sizes are all over the place. 5.56mm 7.62mm .338 caliber, 30-60, 30caliber etc.

Why not do something in even increments like 5mm, 6mm, 7.5mm or 30, 35, 40 etc calibers? Is this just for companies to get extra money making an odd proprietary size? What gives?
gpt
They're just different scales measuring the bore of your weapon (imperial vs metric). To explain; 7.62 mm *is* 30 calibre. The calibre of a weapon is the diameter of the barrel, and 30 calibre is .30 of an inch across. 0.30 of an inch is 7.62mm. 

The different calibers are about range and power. A bigger bullet will go further, faster and harder, but will need more powder to do so. They were standardised when inches were the normal unit. Now, we make new weapons with mm rounds, but the legacy of the old inch based rounds remains.