James Calhoon's websiet he posted a good read on primers. It is a reprint of an article in 1995 Varmint Hunter. They do an evlauation of several major primers and discovered that all primer materials are not the same thickness. The thickness of the metal in the primer cup listed a Remington 6 1/2 as .020. The thickness of the 7 1/2 is at .025. Just enough that it might make a differance.
I don't have the websiet handy, but if you do a search for James Calhoon, he's a bullet maker, he has several good articles on the site.