ORIGINAL: trailer
ORIGINAL: UThunter
To get real technical, you measure outside to outside and subtract one caliber (in the case of the 30-06, you woulod subtract .308 inches) but it's basically the same thing.
Personally I find it easier to measure outside to outside on the same side. You don’t have to subtract anything.
Of course if all of your bullets are making one hole as in the case of 1/4" groups using .308" bullets you have no choice but to measure outside to outside and then subtract the bullet diameter.