You're right. Zinc, aluminum, and copper in various proportions make a series of alloys called Zamak. These cast extremely well, and in fact are formulated as casting alloys. And yes, you can make bullets out of some of them as you have found out. However, zinc and lead mixtures do not work that way. They form a nasty mess that is lumpy, very sticky, and will not cast for beans. Keep zinc out of your lead!
BTW, this weekend I melted down several large boxes of wheel weights. Sure enough, I found maybe a half dozen zinc ones and some steel ones. Keep your melt as cool as possible and do not allow the zinc weights to melt or you will be sorry!