Raimey,
I hesitate to say " always" because that word is so final. The Germans usually used the bore "not groove or bullet" diameter, but the Austrians usually used the groove (or bullet, not always the same) diameter. Sometimes case length of Austrian cartridges is shown as longer than a similar German cartridge would be, because they sometimes add a mm for rim thickness. I wouldn't expect barrel measured at 6.8mm to slug at .257" (a German 6.8 would represent bore diameter, an Austrian 6.8 would represent the groove diameter, slugging a barrel gives the groove diameter). This is why you should "slug" the barrel to determine the diameter of the bullet you need to use and don't depend on the marking for that.
Mike