I have a .303 caliber British No4 MK2 that I'm thinking about casting for.However, I'm at a loss as to what her groove diameter actually is.She's a 5 groove with a bore diameter of .302 and she slugs out ( measured from top of land to bottom of opposite groove) .307.Several sources say to double the groove depth( .005) for a groove diameter of .312.This makes no sense to me as the widest measurement in her barrel is .307. Recovered jacketed bullets , with enough shank to be measured , come in at .307 just like the slug.Have tried .312 Hornadys .311 Speers and Sierras and a box of Winchester factory loads which measured .310 and accuracy seemed to improve as bullet diameter decreased.I'm kinda confused, any help appreciated...