I should explain my question more fully:
I have a couple hundred pounds of 25# lead bricks, alloy unknown. They're a little harder and more silver than pure lead, purchased to ballast some equipment, but I don't know from whom. I was resistant to taking up casting, so I've shipped a couple boxes of them to other forum members in exchange for some finished bullets. One reported that it just needed a little tin added. I hit one of those bullets a couple times with a hammer and flattened it to less than half its original diameter without any cracking. I guess this rules out a zinc problem?
So, I'm reconsidering casting some bullets myself. My spousal critter is opposed to using the oven to heat-treat lead, so I'm waiting for a chance to try that on a sample of my lead bricks when she's not around.
Assuming they don't harden when water dropped, my plan was to try something like this: melt one brick (25#) with half a pound of tin, two pounds of 70%lead/30%antimony hardening alloy, and something like 2.5 pounds or 5 pounds of the magnum shot (which amount is the question of my OP), make ingots from that and test water-drop hardening of that alloy (which, if my calculations are correct, should be about 2% antimony from the hardening alloy + a little from the shot), so that should work if there is enough arsenic, right?
Assuming that works, I'd probably try mixing that alloy and the original brick alloy one to one and test heat treatment again. If the second alloy failed to water harden, that would indicate that my bricks have little or no antimony, or I'd run out of arsenic.