Sounds like lead to me, but others may have a different thought. You say you've been shooting cast bullets, so I'd say that is what it is. I would get a solvent that is specifically for removing lead and work with that.

If that doesn't work, is there a gunsmith nearby that can remove it for you? You have 6 weeks so maybe there's time to get it done.

Are you saying the gun does not shoot acceptably now?