Stick-on wheel weights are soft, but not necessarily pure lead. Clip-on WWs can be lead/antimon/tin mix, zinc, or iron. Lead alloy WWs will bend and are easily marked with side cutters - the others, not so much...
I've had cable sheathing that was pure lead or nearly so, but it was salvaged from an old power plant. Telephone cable sheathing that I've had contains other things including antimony, tin, and copper at minimum. ANYTHING that floats to the top of the pot as the melt comes up to temperature should be skimmed off prior to fluxing IF SOFT "LEAD" is your desire.
"Hardened lead" as was used by "white hunters" in Africa contained a few percent of antimony or maybe 5% tin, so weren't terribly hard. WWs with nothing added are harder. Linotype contains too high percentages of antimony and tin to be of use in even smoothbores unless you want balls that will shatter.