Tough one to answer without seeing what's up. Could be that you're seeing some minor burnishing, could be covered in carbon.

Some bullets are very soft, others not. The bullet going faster likely has less to do with it than jacket material, powder choice and throat wearing in.

Originally Posted by BrentD

I would not buy something that runs on any kind of primer given the possibility of primer shortages and even regulations. In fact, why not buy a flintlock? Really. Rocks aren't going away anytime soon.