Ooh look up dum dum rounds, there was a YouTube video I watched recently, but if you like firearms it’s a fun watch.

So iirc, lead is so soft that when we started using auto feeding the lead deformed and you would fail to load a round. Copper is used because it is softer than the steel barrel so the rifling will grab onto the bullet, but hard enough not to deform with normal use and in the barrel.

But then you find out that they are bad at killing humans.