Joe
I'm guessing you are likely wanting a full octagon barrel. Unless I'm mistaken (and I am frequently according to my wife) with all the makers except Green Mountain, that will be an extra expense to get octagon from a round barrel. I have several Green Mountain barrels in use and I've always been please with their performance.
I would do as Bob Saathoff recommends and stick with the modern bore diamters to eliminate all the problems associated with finding molds, etc. As far as brass is concerned, Rocky Mountain Cartridge makes both 40-50 SS and 40-70 SS although they are kinda pricey.
Phil
