Handgun bullet casting for fun and profit!

[Mr. 16 gauge]

If cooled quickly would they be hard enough?

If it is pure (or almost pure) lead, probably not.....you need to have some antimony in the mix. I usually get mine from a handful of lead shot thrown into the pot. You can also add tin as well, although that is getting pretty scarce (and pricey).
I don't remember the exact chemistry/physics, but Lyman's casting handbook had a good write up about how the antimony helps harden the mix when it is cooled rapidly.