It's based on the time honored .303" reference boolit the bore riding Lyman 314299. This GB mould will drop .305" on the nose and .316" on the rear driving band.
Phwoarrr!!!
It is being made by Night Owl Enterprises who deliver quality moulds to a + or - 1 thou tolerance. I know, I've got a few of theirs so speak from experience.
Eat your heart out Lee "Precision".
As always MAH has no connection with this GB and as a site does not endorse this GB, I do personally though.
http://castboolits.gunloads.com/showthread.php?t=61892
