You gotta dig deep to find a .318" bore. The three commission mausers I had were all .323" as they had been converted in later years. You would pretty much have to stumble across an original, never used military rifle - or a sporter that has been set up as .318" from the get-go. Considering all the wars since the bullet specs changed to .323" it is extremely unlikely. Pretty common for mausers to be built, rebuilt, and rebuilt some more.
I'm not saying they don't exist, I'm saying they are very, very uncommon.