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Old March 15, 2013, 08:04 AM   #11
Bart B.
Senior Member
Join Date: February 15, 2009
Posts: 6,318
Bore diameter for the .308 Win. is .300000000000000"

Groove diameter for the .308 Win. is .3080000000000000"

Same for most 30 caliber cartridges' barrels originating in the USA.

A 30 caliber rifle was originally meant to mean that its bore diameter was 30 one-hundredths of an inch back when one caliber equalled 1/100th inch. Note the absense of a decimal point in such terminology; 'twas correct English at the time. But times, the use of English and people change, so these numbers get muddied up a bit. Remember the 50 caliber machine gun? It's bore diameter was, and still is, half an inch; exactly 50 calibers or .50000000000"

To further muddy the waters, 30 caliber bullets have been made in diameters from .3065" up to .3095". And 30 caliber barrels' bore diameters have ranged from .2970" up to .3030" (.3030" acutally measured in a 30 caliber Douglas blank). Groove diameters have ranged from .3065" to .3100".
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Last edited by Bart B.; March 15, 2013 at 08:15 AM.
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