I use lead, but mine are hard cast. The best thing for you to do is clean the bore after each match with a product called "Lead Away." It's a rag soaked in a chemical. You cut a piece of the rag off about the size of a cleaning patch, then run it through the bore a few times. The chemical softens the lead, and will push out black junk (lead) that you didn't even know was there. Run a clean, dry patch through it after the lead-away patch and you're golden.

The stuff works great.