Factory ammo for 9.3x72R is loaded with a bullet that is useable in barrels of differing diameters, essentially it only has a "driving band" of full dia.This doesn't drive the pressures up to dangerous levels.A barrel marked 9.3, and not marked under the 1939 rules,would have around .375"groove dia.See discussion above about the cartridge with "outside lubricated" bullet. Many of these old rifles have an odd number of grooves.This makes measurement of groove diameter very "iffy".
Lacking a special "V" anvil mic, the best way is with a "V block".If very much care is taken and the slug is rotated against the jaws of calipers, a pretty close measurement can be made.With an odd number of grooves, the "land" on one side of the slug is opposite a "groove" on the other side.
Mike
