system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why do all modern bullets require a casing? Isn't that just a waste of resources (metal)?

Not a gun person so I'm not sure. Looking for info, please feel free to point out things that may be obvious to someone who isn't educated on guns.

EDIT: Maybe I need some clarification on how a bullet actually fires in a modern gun. Not afraid to admit that.
gpt
The casing keeps the propellant inside so you don't have to load it like an old timey musket by placing the powder in and then place the bullet in.

Caseless ammunition has been developed but it's been a bit disappointing and doesn't work as well as was hoped.