Burbank Jung,
If you have a good quantity of cast bullets in the mix you will be good to go. As far as jacketed bullet cores go, Sierra says they use five different core alloys, depending on what the bullet is designed to do; pure lead, 1½% antimony, 3% antimony, 6% antimony, and 4% tin. So if you have a good percentage of hard cast bullets in the brew and you have cores melted out of jacketed bullets and mix the two about 50:50, you should still have a very decent casting alloy for target loads. It won't shine as brightly as some with additional tin, but as long as you aren't worried about looking like you are shooting silver bullets, it should work fine.
I think this really belongs in the casting forum. I will move it for you. You may get some more comments there.