...and Woddleigh caution not to use their .423 bullets in a .418 bore....Woodliegh reloading manual has this to say
"When, first designed, the barrel specifications called for a 0.410" bore with a 0.418" groove diameter firing a 0.422 bullet. The chamber throat was designed to size the bullet down as it entered the rifling." Apparently this design was soon changed in favour of the bullet matching groove size.
As a side note Woodleigh FMJ's have a fairly thick steel cup and are .0005 to .001" undersized.