I remember that too, Wade. It did not happen to me. I also recall that folks, maybe not here, but on a cast bullet forum
I used to participate in, stated they got the tar-like substance from using bore-butter & that it would build up and needed
to be removed with break cleaner or some-such chemical.
I have not had ANYTHING build up in my bores since I learned to use snug combinations, in about 1973/4. For something to
build up in the bore, would be an impossibility. My ball and patch combinations go right into the corners of the rifling with lots
of compression - and - easy loading.