Dang. I have about 150 pounds of wheel weights I had intended to cast into muzzleloader balls and bullets.
They will work fine for balls. Even in revolvers, so long as you use a loading press and load the cylinder off the gun. There are in fact "lead free" balls that are much harder than wheelweight alloy sold, and in use, where the "environmentalists" have been allowed to run wild, and they have proven to work, albeit not as well as lead for a variety of reasons.
Wheelweight alloy is not so great with REAL bullets or Minies, as with REAL's (Rifling Engraved At Loading) they are very hard to load and with Minies, the harder lead does not work as well for the skirt to flare and properly engage the rifling.
The only thing you need to be aware of is that in recent years, driven by the false hysteria over lead, there have been a variety of non lead wheelweights used, including some made of zinc and some of steel.. Normally you can tell with fair accuracy just by how soft and easy they are to scratch, and after a while you will be able to pick them out mostly just by sight.
Steel is not such a problem, as like the clips on lead wheelweights, it just floats to the top. zinc however you have to be very careful of not to get into your melt as it will muck things up royally, into a clumpy mess that is hard to clean up.
Although zinc, with its relatively low melting point can be used itself to make projectiles, albeit very light ones that are nowhere near as useful as lead, but you never want to get it near your lead casting pots and equipment