You're kidding right? That's not where the difference lies.
You're kidding right? That's not where the difference lies.
In .308 and 7.62X51 (and the .30-'06 and the .30-'03 and the .30-40)
all (at least for all practical purposes) cases it is a nominal .30 bore and .308 groove. Remember there are non-standard rifling forms - e.g. Microgoove with larger bore and smaller groove sizes - and differences in measurement depending how far one chooses to carry the decimals.
Putting very much weight on the artificial conventions of naming gets pretty silly. FREX the .218 Bee, .219 Donaldson, .220 Swift, .221 Fireball,. 222 Remington, .222 Remington Magnum, .223 Rem, .224 Weatherby, .225 Win can all use the same bullets though the cases are quite different. It would be a great mistake to take the different numbers as meaning the bore and groove diameters must be different.
FREX .30-'06 and .308 Winchester refer to the same nominal bullet diameters one referencing bore size and one groove size. Individual bullets may be made oversize in hopes of better accuracy - some Lapua - and cast bullets generally run larger than jacketed.
In most cases (Lake City Brass
) any distinction is without a difference. However the SAAMI specs and the military specs are incommensurate and the military headspace is gaged differently and some military firearms run long chambers or have excessive headspace by SAAMI gages. Some non-military production, it's been reported of the Steyr Scout, may have long chambers in hopes of enhancing feed with military or just dirty and corroded cases or otherwise working better under adverse conditions.
Jerry Kuhnhausen in his gun smithing books, Clint at Fulton Armory and many more in print and on the web expand on all this in great detail with diagrams.