While doing my daily check of classifieds, I cam across a guy selling a Deerstalker that had a half-round barrel. I have never seen such a gun. Special order/distributer run?
I had a TC Renegade with a half round barrel, I found out later that the barrel was an add on by a gunsmith friend of mine who had a stock he wanted to make a gun out of.
The Lyman deerstalker as it came from the factory had an octagon barrel as far as I know. It was marketed mostly to hunters. You might try posting this on the sister site aimed at hunters and in-lines even though the deerstalker is a sidelock. May get more responses.
In the early 1990's, Investarms made 20ga. shotguns, with screw-in chokes no less, for Navy Arms. Perhaps one of the bbls. found its way to Lyman as a .50cal. smooth rifle? Obviously this would be prior to reaming it to 20ga.