My learning curve was the reverse of your's, as I started with wheelweights for pistol bullets, and cast my first round balls with pure lead just a couple of weeks ago. I had to chuckle when I read your post, as your confusion mirrored mine
. OK, what makes wheelweights different is antimony, not tin, and you can't get it out by any method that reasonable for the home caster. You're not waiting long enough if your sprue cut-off is brittle. Yes, I waited and waited for the sprue to harden on my first round ball casts, before I realized it was already as hard as it was going to get. You don't need as much heat for wheelweights, as lead solidifies as 621°F and wheelweights about 580°F, varying on the antimony content.
I've shot wheelweight round balls in my flintlock, as well as the Hornady and Speer swaged lead balls. Wheelweight balls are much harder to load with a too tight patch. Otherwise, no real problems.
If you don't want to cast with the wheelweight metal, find a pistol bullet caster and work a deal.
Here's a post on wheelweight analysis. http://www.shootersforum.com/showthread.htm?t=14155