ligonierbill,
It is highly unlikely that the Stiegele is chambered for any American caliber, and unlikely that is the Danish round. The most likely caliber would be the 11.5x50 R Werder, as you supposed; or one of the very similar variations of this cartridge. If so the rim recess should be deep enough to accept the 2.40mm rim of the 11.15x60R Mauser( Mauser Base, or MB) case. Mauser Base cases are no longer readily available( read "cheap") here. I have a similar Buechsflinte ( the German name for this type gun) chambered for a similar cartridge that uses a .446"bullet, and a friend has a bolt action single shot, chambered for the Werder cartridge. Both of us make highly satisfactory cases from 45-70. To do this, we move the outside part of the rim forward to approximate the Mauser base rim, reduce the rim diameter to closely fit the rim recess( in my case .595"),then trim as appropriate. Depending on the maker of the 45-70 cases, the new cases will be about .015" undersize. This is acceptable and will expand to fit in fire forming. Fitting the rim to the rim recess centers the case at the rear, and the bullet does so at the front. This ensures an even expansion of the cases. When you receive the chamber cast, let us know the particulars and we can visit the question again. This type rifle commonly has a barrel with an uneven number of grooves, which complicates measuring the groove diameter.
Mike
