Oh, come on, guys.
Everyone always ignores the great-grand-daddy of so many 'Merican cartridges.
.30-03 (inspired by the Mausers) is the parent for .30-06, .270 Win, .280 Rem, and the Whelens (.35, .375, .400). - And none of the latter three are based on the .30-06. That's why they require a .30-03 neck length for proper forming.
(Edit: Now I see that Mike brought up .270 Win being based on .30-03, a bit earlier.)
The .280 Remington could be argued to be based on the .30-06, if you include elongating a case to be 'based on' one - i.e. .308 Win being 'based on' .300 Savage by being a stretched and modified version of it, or .30-03 being 'based on' 7x57mm by being an elongated and modified version of it.
But, as far as I'm concerned - if you add length, you're creating a new cartridge.
If you can't take the 'parent' and form the 'child' directly from it... it ain't part of the family.
