Shot contains a smidgeon/trace of arsenic. Arsenic is needed for hardening the castings. WW also contain the requisite arsenic for further hardening if desired.

When casting without tin (tin does not contribute to the hardening process, such as antimony and arsenic does), the temperature of the melt and mould must be higher when casting. I like an even 'frosting' of the cast bullets, which has worked well for me since the 70's.

A bullet of simple lead and tin mix cannot be hardened.

If dropped into water from the mould( NEVER allow water to splash into the melt) bullets cast from straight WW allo (not zinc WW) will attain hardness from about Brinel 25 to 32. Good clean, new Linotype metal is brinel 22. Old oft used Linotype is said to be from 20 to 21 brinel. Linotype is quite brittle, but hardened WW alloys are not.

Shot may contain up to 5% antimony - apparently, but 3% is most common for "hard shot".