Caliber size metric vs. standard question

[CRMarks]

Can someone explain why a 6.5 mm caliber is a bullet diameter of 0.264" in the standard unit system? Since the metric to standard conversion is 25.4 mm = 1 inch, I keep getting that a 6.5 mm bullet should be 0.256". Am I missing something on bullet dimension measurements? I'll confess to not having any experience in handloading and I'm certainly no expert in cartridge construction.

 

[p_ham]

.256 bore, .264 groove

 

[std7mag]

P_ham is correct.
The only one i know of doing actual bore diameter is Lazzeroni.

 

[CRMarks]

Awesome, thanks! That has been bugging me for awhile. Learn something new every day on here.