I believe the larger groove dimensions are on the older pre WWII rifles my rifles bore measures .255 (6.48mm) which matches the current CIP standards.

http://www.cip-bobp.org/homologation/uploads/tdcc/tab-i/tabical-en-page22.pdf

I have yet to measure groove dimensions which I expect will be .264.


Dave Weber
doublegunshop.com
DoubleGun Evangelist
In Charge But Not Responsible