Bullets cast from the old WW with the clamping clips can be hardened to anywhere from 25 to 34 brinel by soaking in a hot oven that is set 10F below bullet sagging temp, then quenched in water. If sized and lubed immediately they will harden to between 30 and 34 brinel but will not be brittle like linotype which is only 21 brinel.
The reason for sizing and.lubing immediately, is that the bullets "take" 12 hours to harden. If they are allowed to harden before lube and sizing, sizing them will actually soften them again. They must be sized immediately after quenching.
Hardened lead bullets will pen like FMJs and can be driven to 3,000fps if the lube used is up to it.
Shotgun shot can also be cast into bullets and can be hardened somewhat as it, like the ww's, contains trace arsenic which is necessary for the antimony alloy to be hardennable.