Wheel weights have antimony in them,it makes them hard and brittle.Bullets cast from wheel weights will be brittle.The longer they set up after they are cast the more brittle they become for up to 6 months is what I want to say but I'm not sure anymore about how long but the sooner you shoot them the softer they will be.After a 6 months or so they won't get any harder.There is a book called the Art of Bullet Casting,its several years worth of articles from Handloader Magazine.It can be found at amazon.com.There s not much about muzzle loading but there is a lot about wheel weights and other alloys for casting bullets.I shot hand gun silhouette for a while with wheel weight bullets,the velocityof 44 mag 245gr. bullets isis close to 54 cal round balls out ofmuzzle loaders.They were very brittle.Soft lead tended to flaten out,wheel weights tended to breakup.OK for targets,bad for hunting.Wheel weight roundballs work for practice at the range,but the breakup on bones like shoulder blades.Had one out of a 44 mag breakup on a feral dogs head once at about 30 yards.Killed him again 6 months later with a 308 Norma mag,had a nice scar on his head just above one eye.Lead and tin would work to cast a harder bullet or ball but not wth antimony.You can't get wheel weights hot enough at home with a electric melting pot orpropaneto seperate the antimony and tin from the lead.