Generally speaking, most wheelweight metal is too hard for muzzleloader bullets and round balls (but there is some variation in wheelweight composition!!). Pure soft lead is what is called for. If your bullets are the right size for your bore, anything harder makezs bullets that are difficult to start into the muzzle. In addition, if you use anything harder than pure lead, the bullets tend to cut the patches when loading, and do not "obturate" or upset enough to swell up into the rifling grooves enough. Balls cast from wheel weights might be OK for cap & ball revolvers, if your loading lever has enough mechanical advantage to seat them to the right depth in the chambers.

Keep yore powder dry!!