I thought lead is softer. Why is it considered better for ammunition?
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Lead is good both because it is very dense and weirdly because it is soft.
Most modern firearms use something called “rifling” which is where the term “rifle” for the class of firearms originates from. They are a curving set of grooves inside the barrel which bite into the bullet and cause it to quickly rotate as it leaves the barrel. This aids in maintaining accuracy. A softer metal such as lead or copper allows the bullet to deform to match these grooves instead of just tearing the barrel up.
Of course the main goal of the bullet is to transfer impact energy to the target and being dense helps with this. Think about throwing a pebble at someone vs. a foam peanut, it matters a lot how dense the projectile is. Lead is very dense and fairly common, which is also a great feature. You don’t want to be throwing difficult to find materials downrange because chances are you aren’t finding them again.
Lead is also easily formed. If someone wanted to cast their own bullets they could put a pot over a campfire and melt down lead to pour into an iron mold to create new bullets. Lead melts at 621 F compared to copper at 1,984 F so this wasn’t really possible for other suitable metals.
Finally lead deforming on impact isn’t a bad thing because it would tend to “mushroom” and expand to slow down, dumping impact energy into the target quickly.
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