Was the crud a brownish almost ear wax looking stuff? If it was, it's old bore butter. And yes, I would bet that is an accumulation of years of use. That's why the bore butter users I've noticed keep insisting to scrub that barrel back to bare metal before they reapply the bore butter again.

You might be very pleased the next time you take it out shooting with the accuracy. I had the same thing happen to me many years ago. And after that I stopped using bore butter. I've also often times used the next bigger brush size in bores. Especially when they get worn down a little from the other bores. Then they fit and work real nice..

Sounds like a real nice range rod you got there.