Best thing in developing tight groups is slugging out the barrel, although measuring a five groove barrel can be a bit tricky as no two grooves or lands line up across the diameter of the projectile.

Then me thinks why not fill them in... insert the slugged lead into a low speed drill, drill into a block of soap or wax, remove and then measure.

Found that .001" over groove size worked best.

Best moulds come from CBE in Australiaicon and he has a new range for .303 Brit.

CAST BULLET ENGINEERING Bullet Moulds Molds