The 270 Winchester and 7x64 both use bullets smaller than the bore diameter marked on the barrel (also the 270 wasn't introduced until 1925) so neither of them is indicated. The indicated bore diameter of 7.4 mm means the actual bore is between 7.4 and 7.5mm (a 7.5mm gauge would not enter, but a 7.4mm gauge would). A bore diameter of 7.499mm would be pretty tight for a 30-caliber rifle but we often see tight bore diameters. Raimey is correct about the meaning of the proof marks, but they do not show barrel groove diameter, chamber neck diameter or case length. A chamber cast (and maybe "slug" the barrel) would be required to determine these dimensions. If these dimensions are verified as correct for 30/06 the question arises, "why doesn't the bolt close on a go-gauge?". The bolt may have been swapped out during the last hundred years or the barrel may have been installed using a cartridge as a go gauge (without allowing for tolerance). The barrel and action are both numbered and if the bolt doesn't match (may only have the last two numbers), a different bolt may have been used. The rifle was built using a surplus WW1 action, so different numbers don't positively mean the bolt was changed. There is no way now to prove how the barrel was installed. I suggest actual cartridges be checked to see if they will chamber (some brands may chamber and others not). If a factory cartridge chambers (after verifying other dimensions), enjoy using that brand. The problem with headspace comes in if there is grossly excess headspace. Having minimum headspace is not dangerous. Having a too small chamber neck diameter or too short chamber neck area can possibly be dangerous if the case can't expand to release the bullet (a fired case should easily accept a .308" bullet). Good luck.
Mike
Last edited by Der Ami; 08/15/24 02:12 PM.
