Mike, what make of gun is this and where was it made? If it is a German or Austrian you may have to dig thru Dixon's "European sporting cartridges" to find the answer. Take your chamber casting & start with the rim dia. & flange thickness. A lot of the German rounds were different with a taper right in front of the rim. If you don't have the book, pass along all case dimensions & i will take a look.

m-4