Tin itself has a fairly high BHN number.
I don't remember it for sure but 35 comes to mind.
of course it being cut in half with pure lead will drop it.
the other part of it is tin only lead alloys will age soften over longer periods of time.
Hardness from aging or water dropping may come and go but as long as the alloy hasn’t changed it can be brought back.
Hardness testing ingots is a fools errand. The bullets cast from the ingot will cool at a vastly different rate and that affects hardness. Heat treat them and it matters even less What the ingot tested for hardness.
tin only and high tin content alloys do not react [well] to speed quenching.
the quenching works on antimony.
think about it like the antimony chain is broke apart you speed cool the alloy and those pieces of antimony are spread out and not tied into a long molecular chain.
they are frozen into place affecting more of the lead because they are now touching more lead through more surface area.
tin is on the surface and retards the effect, plus it has a different shape to the molecule so it just is as it is.
12 and 18 BHN are a gimme with the 2.5% alloy I mentioned earlier.
dropping it down to 1% tin and 2% antimony should put you right where your looking [or close enough nobody's gonna know the difference]
you can add tin until your blue in the face but it has it's limits and it isn't BHN adjustable except for adding or subtracting more 20$ a pound tin.
that just isn't cost effective in the least when you have to add 100 or 150 dollars worth of tin to every 100 lb batch of alloy.
if that 6% [antimony? right??] alloy has been subjected to stress or pressure the BHN will be low.
if you melt and cast with it you will see close to 15 bhn air cooled.
I know it's confusing, but just because antimony is present it doesn't necessarily mean you will have a high BHN number.
also your alloy can vary slightly from batch to batch in final BHN numbers. [even if it is foundry certified]
so 1-2 BHN is meaningless,,,, it really is.
I try to keep my stuff in a window just in a window, remember that is the whole principle behind the Audette ladder test.
your looking for a window of powder loads that will give you the best chance of close-nuff being good enough for the rifle your shooting.