I dug out my NRA Cast Bullet Handbook. It was copyrighted 1979 & was a compilation of articles pertaining to cast bullets which had appeared in the pages of "The Rifleman" magazine between the years 1957-1979, edited by E H Harrison. As the entire book was devoted to bullet casting, I hardly think any warning therein was simply "Generic" in order to dis-courage bullet casting. The danger stated was that at time of writing Lead/Antimony/Arsenic alloys used in battery plates were often (particularly in maintenance-free batteries) being replaced by Lead/Calcium alloys. To make a long article short, 1st the Lead/Calcium alloys were stated to be "Ill-Suited" for bullet casting purposes. Next the "Danger" factor was when the Lead/Calcium alloys became mixed with Lead/Antimony or Arsenic alloys the possibility of Acute Poisness gases Stibine or Arsine became Very Real. Both of these gases were stated to have high density relative to air & would accumulate in low lying places. This may well apply only to US produced batteries, but would make certain before melting down any battery plates.
As to the hardness formula mentioed a comparsion of two alloys can be done easily with a sample of each alloy having a smooth surface. In the article they took a pipe cap, bored out the threads on an angle (for easier release) & faced the bottom to a very smooth surface. ( was stated to be better to have a smooth surface as cast). A ball is then placed between the two surfaces & squeezed in a vice to no more than about , ¼ it's dia. A fairly large ball makes measurement easier, but is not necessarily a requirement. The dia's of the indents are then accurately measured & the hardness is inversely proportionate to the sq of the dia's. If you had for instance a pure lead control piece of BHN 5 & the indents measured 5mm & 4mm, then 5BHN x (5/4)² = 7.8BHN etc. Even if the BHN is unknown this can still give relative harness compared to an existing alloy.
