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Mmm, thanks Rule, then I gues I should have being referring to the " not true" 7x57, " not true" 7x64....sorry my mistake.

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Yep, the 277 is 7.014mm, the 7X57 etc 7.24mm. Nitt picky I know.

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As I see this,

The .270 has a .277" groove diameter. The .276 (7mm) has a .284" groove diameter but .276"/.277" bore, hence the Brits named it the .276, not .7mm or .284.


The same? - not a chance, but they are as close as a .358 is to a 9.3 (.366")

and a 9.3(.366") to a .375.

All 3 of these comparisons use the former's groove diameter as their bore diameter.

What changes the most and usually more than simply bullet diameter, is the bullet weight in the normal run of 'factory' ammo.

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Yep I was talking grove/bullet diameter. Please do not confuse "The 276" with an actual measurement of .276" "The 276" is a false 7mm ie 7.24mm, where as the measurement .276" is a true 7mm measurement.