taken from: From Ingot to Target:
A Cast Bullet Guide for Handgunners
Heat treating and water quenching - This age hardening of antimony containing alloys can be accelerated at higher temperatures, i.e. heat treating the bullets. This is most commonly done by sizing the bullets first (since lead alloys work soften, and hence sizing would negate a significant portion of the hardness imparted by the heat treating process) then heating them to about 450o F in the oven and quenching by dumping them in cold water. The hardened bullets are then lubed using the same sizing die that was used before (so that no actual sizing takes place). Done in this manner, bullets cast with an alloy containing 5% antimony, 0.5% tin and 0.17% arsenic, which would normally have a Brinell hardness of a little over 16 (after aging for 6 days), can be hardened to a BHN of over 35 (see Dennis Marshall‘s chapter “Stronger Bullets with Less Alloying“ in “Cast Bullets” published by the NRA). Notice that this alloy is not tremendously different from the common wheelweight. Much the same sort of result can be obtained by casting with a hot mould and water quenching directly from the mould (place a towel over the water bucket with a 4” slot cut in it to contain the splashes). Mould temperature is critical for maximum effective hardness. Bullets water quenched from a “cool” mould (i.e. one from which the bullets were smooth and shiny) were found to be similar to air-cooled bullets. But bullets dropped from a mould that was “hot” (i.e. hot enough that the bullets were frosty over their entire surface) were found to have BHN of over 30 when water quenched. In a separate study, such a mould was found to have temperature of 430o F, very similar to the optimum oven temperature found in the heat treatment study (ca. 450o F). I don’t normally cast quite this hot, but even so, water-quenching WW alloy routinely gives me bullets with a Brinell hardness of 18. One of the advantages of hardening bullets in this manner, as opposed to using linotype to make them hard, is that they are tougher and not as likely to shear or fragment on impact.
