[T. O'Heir]

Slug the barrel(should be .510") and make a chamber casting using Cerrosafe(Brownell's).
There were several .500 Black Powder Express cartridges. 1½-inch, 2-inch, 2¼-inch, 2⅝-inch, 3-inch and 3¼-inch. A chamber cast will tell you which.
Barrel are in surprisingly good condition for a 100 plus year old rifle.
When that rifle was made everybody and his brother was making 'em. It's possible that only the action/lock was made in Austria.