G'Day Fella's,

Appologies for bringing this news to you but 6.7mm is actually 0.264" (AKA 6.5mm).
Just as Rothhammer1 has already highted to us, in his above post.

What most people don't realise is, just as the Brits so often refer to the Bore Diameter of a rifle's barrel, so do most cartridges, conceived or registered (NATO) in Europe.
So what is refered to as a 6.5mm barrel, has a Bore Diameter of 6.5mm or 0.256" and barrel Groove Diameter of 6.7mm or 0.264" (the same as the bullets).
That's approximately 0.2mm or 0.0078750" (lets round that up to 0.008" or "Eight Thou" ..... of an inch) difference, or 0.1mm or 0.0039375" / Four Thou each side.
Remember the .256 Newton cartridge (I think thats the one/its title?).

It is the same with the Good Old .303 British (0.303" and 0.311" ish) and another the 7.62mm x 51mm.
In the case of the 7.62x51mm NATO, the barrels Bore Diameter if 7.62mm or 0.300 and the Groove Diameter is 7.82mm or 0.308" (the same as the bullets it shoots (well, 0.307" Military to 0.308" Civilian).

If any of you blokes own a pair of Digital Vernier Calipers, that can switch between Metric and Imperial. Dial up the Imperial bullet diameter of your Pride and Joy, then press the Metric button FYI. ............

So the Groove and bullet diameter of your 7mm is 7.2mm and you 8mm is 8.2mm etc ...........

I hope todays Engineering Lesson from your favourite Poor Dumb Fitter, has been of some assistance to you.

So ............ nowyoucanAvagreatweekendeh!
Homer