It is reproducible if ya work with consistent alloys and consistent technique.
Yes. As was written earlier, it ain't rocket surgery.
Most alloy in the "clip-on wheelweight" category can be toughened to near 30 BHN maximum, heat treat to just below the slump point in a convection oven for one full hour (find the slump point the "hard" way), go straight into a bucket of cold antifreeze without passing go. In a few days it will be close to as hard as it will ever be, though it takes another week or two to settle down fully.
Water-quench from a hot mould, cutting the sprues at the instant they are solid enough to not smear but will leave a goat-arse pucker on the base, you can expect 24 bhn in a few days. Cut that alloy in half with near pure lead and you still get 18-20 bhn, but it takes about three weeks to get there. Add a bunch of tin to either and the PH process slows down while also changing the fundamental shooting characteristics of the alloy for better or worse, depending on what you intend to do with it.
If you use "pure" alloy from a foundry, you'll likely have lower numbers because of lack of trace grain refiners, principally As, but the reward is consistency from batch to batch, year to year, if repeatable exactness is of particular importance to what you're doing.
I've cast literally tons of WW alloy, from various sources through the years, often alloyed with soft-ish scrap sourced from roofers and plumbers, and found a great deal of consistency in my bullets when using any given hardening technique. I can usually count on exactly 19 BHN from .30-caliber bullets cast of 50/50 WW/soft Pb, water-quenched from a hot mould into lukewarm water, after the bullets age three weeks. Air-cooled, the same alloy ends up about 10 bhn after a month, regardless of caliber. If I add 1% tin to the air-cooled, it ends up about 11 bhn.
Part of the thrill of all this (for me, anyway) is scrounging scrap alloy and tinkering around until I can get what I want out of it. There is a little bit of pastry chef involved, but the pie recipe will turn out plenty good if it's close, and if it isn't, it usually doesn't take much head scratching to get it right.