The double rifle you refered to elsewhere is (perhaps) a .318" Westley Richards, and the .318JR Mauser you refer to above is, I'm sure you already understand, a different cartridge.

If the rifle is German made and proved in Germany between 1912 and 1939 it will be marked something like this, "St.m.G 15g". This means Stahl Mantel Geschoss (stell jacket bullet). The bullet weight shown in grams can be multiplied by 15.47 to get the grain weight, in this example (on my 1924 Drilling), it's a 231 grain bullet.

The early military .318" bullet was 14,6g or 226 grains.
The sporting RWS H-Mantle bullet was, by comparison, 244 grains.
The standard .318" sporting bullet was 12,70 grams or 196 grains.

If there are European proof marks on this rifle that don't match these examples, that you need help with, post a picture and I'll do my best to help. A chamber cast would also be a very good idea, easy to do and should not harm the gun.