Very few out there are still .318"
When the Germans went to the larger bore size (and new bullet), they pulled in all their military rifles and had them converted to the larger bore. Just as we did with the m1903 Springfields in .30-03, we converted them to .30-06.
Naturally, as these things go, a hand full of rifles always seem to get missed.
And, on top of that, there were German sporting rifles that stayed in the .318 bore, and ammo was made for them for many years after.
If its a German military rifle (or ammo) odds are long it is .323, BUT, since a small chance exists it could be .318, it pays to check.
All else being equal (and it almost never is) bigger bullets tend to work better.