John L
WELCOME TO THE FORUM ! !
Thanks for your reply to the questions.
It is my opinion that the " hub " of your problems lies with the plain base bullets at .309". The bullet must fill the throat properly ( May take a .311 " dia. cast bullet to do that ). At .309", most likely hot gases are skirting around the base of the bullet causing the majority of your leading.
It sounds as if your barrel is clean enough to shoot cast bullets cleanly right now. You've just got to change things a bit in your loading sequence.
Do you own any molds and do you cast your own .30 cal. rifle bullets yet ?
Many of us on the forum shoot 100 rounds or more of our cast loads in .30 cal. rifles without a hint of leading.
Some of that black that you're seeing is graphite and carbon, it is extremity hard to ever get all the carbon out of a barrel, ( I don't even try ).
Some rifles will shoot with Lee Liquid Alox , some won't.
I'd like to see that barrel clean and you load something like the 311291 with a good lube ( Ben's Red comes to mind ) and a gas check , sized to .311".
I'm betting that you could fire 50 rounds of the 311291 load, then push a single clean white cotton patch through your barrel and it would look mirror bright.
Ben