Years ago (many years ago) when I corresponded with Elmer Keith, I asked for suggestions on the best alloy for slugs in a shotgun or Paradox. He replied that I should mix 7 pounds of plumbers lead with a pound of 50/50 bar solder. I always wondered where he got that recipe.

As for adding mercury, it is tricky to do. It fumes at the melting point of lead, so you really should do this in a fume hood. I did some limited testing based on old patent and research papers, and was able to confirm that an alloy of:

96.5% lead, with the remainder being trace elements
1.0% mercury
.8% calcium
.8% tin
.9% trace elements (primarily aluminum, copper, silver)

would consistently result in an alloy with a Brinell Hardness of 24. (Pure lead is 5.0 HB, wheelweights are often heat-treated to around 20 HB, Linotype is 22HB, many Babbitt alloys will go harder depending on composition.)

Then I lost access to the fume hood, and went back to the typemetals and Babbitts I'd been using without the worry about mercury poisoning!