Determining groove diameter in odd# barrels

[Double 30]

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...

[curator]

If you tightly wrap your bore slug with brass shim stock and then measure it, subtracting 2X the shim stock thickness you'll get a good idea of the actual groove diameter. Most 2-groove barrels will be .315-316. 5-groove barrels are usually .313-315. Truthfully the throat diameter is more important for accuracy. The larger the bullet, the more accurate it will be provided you can chamber the loaded round. If there is no room for the neck to expand when the round is fired, pressures can go pretty high. Steve Regwell (www.303british.com) has bullets of .313 and .314 diameter. My war-time .303s love the 180 grain .314 diameter slugs!

[Castaway]

Try opening your calipers until you can rotate the bullet betweeen them.