Greeting KanawhaRanger,
According to old Douglas Rifle Barrel Company literature information I still have, their muzzle-loading barrels were cut rifled with 0.010 inch deep grooves.
A 1-66 rifling twist was used in all calibers (32-58) and available in two grades, standard and premium. Octagon bar stock was used and drilled through. The premium grade was guaranteed to be drilled through on center at both ends of the barrel and was stamped with XX,s inside an oval.
Regardless of grade, they were great shooters and many a local, state, and national match was won with a G. R. Douglas barrel. In the hands of a good shot, they are still match winners today. Nobody is handicapped using a Douglas ML barrel today.
G. R. Douglas was the first 20th century barrel maker to produce a production line muzzle loading rifle/pistol barrel at an affordable price. They were the Green Mountain of their day and pioneered the way for the H & H's, GM,s and others of todays market.
Best regards,
John L. Hinnant