Might want to ask Marlin on that one, but usually ported barrels aren't the best to be shooting cast in. Remington bulk jacketed 45-70 bullets can be bought most times,cheaper than commercial cast .
the most expensive bullet there is isn't worth a plug nickel if it don't go where its supposed to. www.historicshooting.com
I had one of the first ported Marlin guide guns and it loved hard cast bullets. I now have a non-ported stainless guide gun and it shoots just a good. I don't believe the ports harm accuracy.
