Gents:

Lee bullet sizing dies are available and can be custom made to adjust cheap and available .323's to .321, .320., .318 etc.

As for "oversize" barrels, Charles Newton in his writings, and Labbett and Meade in their excellent book ".303 Inch", identify the intentional boring/rifling of early-20th Century European rifles to .002+- GREATER than bullet diameter. If this is the case with the Drillings mentioned, .320 or so would in fact be spot-on for .318 bullets. Many European rifles do have "oversize" groove barrels so the sources may indeed be right.

These sources indicate that at least early on, riflemakers determined that there were pressure advantages to such a relationship between bullets and groove depth {at least with the relatively fast-burning powders commonly used then} and with these fast powders, bullet "setup" mated the bullet with the groove depth for good accuracy. Obviously this applies to conventionally-constructed, lead-core bullets, not solid copper, "monolithic"-type bullets. There are some pictures of setup tests in L&M's book that clearly demonstrate this process.

Newton goes on to state essentially that he was one of the first to realize that with modern designed strong, bolt action rifles, no "oversize" boring was necessary.