A TC Hawken had a 15/16 barrel. A 72 caliber 12 ga would leave far too little barrel wall thickness, unless you have a prepaid funeral plan.
Many years ago, mid 1970's some incredibly cheap smooth bore guns with half oct/half round barrels were sold here in the US. Some were advertised as 68 cal and others as 70 cal. very poor quality and as I recall three men were killed and the guns were recalled. (Two brothers were killed in one incident out side Chester PA.) The sporting goods store I worked at had sold them, and took back all but a few. We were paid by the manufacturer to try to track down each gun and even to go pick them up from the owner and give them extra money to get them back. Of course some were sold for cash and couldn't be tracked down.
Every time I hear a person describe a barrel like that, I get the willys. Your barrel may not be from one of those guns or kits, but it could be. Have it checked carefully. These barrels were in the white and the oct part was not tapered at all and went abruptly to round.