It is a geometry thing, got it. If both .260 and 6.5 are at 2.8 inches then the CM cartridge will be just slightly shorter in length than the .260 due to the steeper shoulder angle. And therefore the CM has an ever so slightly smaller charge capacity due to that steeper shoulder angle.
So if Mr. Briggs at NSS is offering a "match" chamber option for the .260 Rem, he is extending the leade length to allow the .260 reloader to seat these longer bullets at their optimal COAL thus negating the CM advantage, at least in a single shot application.
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