Hi, PSE and guys,
Rather than edit the above, I will add a new posting. I see what you mean about the groove depth, given those figures. 6.51mm would be .2563", while 6.46mm would be .2543", or a groove diameter of only .002" over the bore diameter, an impossibility. Somehow, I suspected the 6.5mm designation is only nominal and not a true measurement. So, I miked some 6.5x55 bullets (Swedish GI, Norma, and PMC, and they run 6.66-6.68mm. So, I slugged the bore of a nearly new M94 carbine barrel. Surprise! The ammo makers really do know something we don't - the actual bore is 6.46mm, but the actual groove diameter is 6.66mm.
So if we assume that the actual groove diameter of the rifles is 6.66mm (.2623") and the bore diameter is 6.46mm (.2543"), we have a difference of .008" in diameter, or .004" groove depth, a normal figure.
So why is the disc marked 6.5mm when it seems that figure is a fiction? I don't know, but I would almost bet it was more a matter of just sticking with the nominal number and making the actual gauges the right size.
In other words, Wildalaska may be right, and the disc figures really don't mean much in terms of absolute numbers, though they may be useful in indicating the amount of relative wear.