Yes on Turnbull, and and have McGowan ship them a 20 twist .512" dia octagon barrel in your choice in length, had mine built on an old Browning '86 action, wonderful job they do.
You should be able to get mag tubes at midwestgunworks.com, they also have buttstocks and forends if you run into any trouble trying to use a forearm for a round barrel going to octagon.