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I know this is a dumb question, but here goes:

I have seen references to both 30-06 and 7.62mm NATO as being .308. Yet 7.62mm equates to exactly .300 inch or 30 caliber. Is the .008 allowance for a snug fit in the 30 caliber barrel? If not, why is 7.62mm NATO also referred to as a .308? Thanks.
 

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No, no dumb question's here, if that were the case i would be really dumb.:D Your correct in that 7.62 equals .300 inches which is the bore diameter of these two guns you list as well as the 300WM, 300 WSM, 308Win, 300 Weatherby and many others. The bullet and groove diameter of all of these guns are .308 which is 7.82mm. >308 is the bearing surface size of these bullets. There are others such as the 6.5mm's which is the bore diameter( .256/.257 ) and the bullet and groove diameter for these is .264. On the other hand when referring to the .243 that is the groove diameter and the bore diameter for these would be .216/.217 or 6mm's. The groove diameter for the 6mm's is 6.17mm. Hope this helps !
 

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Yep, not a dumb question at all...most of us have wondered the same thing, when comparing US cartridge designations to European. We typically go by the groove diameter while on the other side of the pond they measure the bore diameter before the rifling is cut. This is not always the case, but it does explain why 7.62mm and .308" are not the same measurement, but are used to describe the same 30 caliber cartridges.
 

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Tobannon welcome.
Keep asking questions, I'm still learning.
If you ask enough questions they will teach you the secret hand shake.

Jim
 

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Yep - two different measurements for same caliber.

30 Caliber Bore diameter = .300" but groove diameter = .308".

Bullets for 30 caliber bore will be the groove diameter of .308".
 

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The Troll Whisperer (Moderator)
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Maybe a little further clarification -

Bore diameter = top of the lands.

Groove diameter = bottom of the lands and the groove between the lands.
 
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