If you check original Swedish Mausers in 6.5mm you will likely find 0.268" dia grooves and a nice 0.256" bore! As yet I have been unable to find out where this "rule" that rifling grooves are only 0.004" deep has come from. Both of the Steyr made Mannlichers in 6.5mm had 0.268" grooves and 0.256" bores as well that is a pre 1924 Schoenneur and a Mdl 1892.