daryl, there is a fundamental distinction between british and german bore rifles. you know that the diameter of a shotgun calibers is not fix but fluctuate. for example in the 28 bore a barrel can go from 13,80mm up to 14,30mm and be in CIP measurments. a german made bore rifle barrel will be with the lands in this range when a british made barrel will have the bore diameter as the grooves diameter.
so a german barrel maker was taken a shotgun barrel and rifled this when his british colleague was drilling the barrel blank much smaller and match the shotgun diameter when ready.