For muzzle loaders you would be best with pure lead rather than wheel weights.
The lead being softer is a lot easier to load and less likely to snap ramrods or loading levers. At these lower velocities it helps that the ball can swell to fit the rifling better.
Wheel weights are ideal for bullets to go in cartridges, they shoot at higher velocities and the harder metal is needed to stop the O/D stripping as it goes through the rifling.
Older weights could be lead but newer ones are harder alloys..
“If a cricketer, for instance, suddenly decided to go into a school and batter a lot of people to death with a cricket bat, which he could do very easily, I mean, are you going to ban cricket bats?” :- Prince Philip said after Dunblane