The twist is 1/28". The bore/groove dimensions are .721/.729 according to ER Shaw. I'm actually having two done. I shipped a takedown Greener GP shotgun to John Taylor Machine in Washington along with one of the blanks. He's going to fit it as a second barrel for the set. The other gun is a former Police Gun. It's going to another smith who is semi-retired and would not want his name mentioned. He has done several guns before and he chambers all his rifled barrels for 3" chambers and the shorter shells shoot more accurately with the longer chamber.