I wanted to echo that one!
Better slug the bore to be sure, or, I have found that you can get a good measurement with a digital caliper at the muzzle. I know .005 in. doesn't sound like a lot, but it is very detectable when measuring.
If you can, just pick up a box of .318cal shells and see how they shoot - if they are accurate, no need to use the "s" bore ammo anyway.