What is confusing to me on some of these cartridge drawings is, usually the major head diameter is given at the point where the case head goes from solid brass ( expands very little ) to where the case has powder capacity (expands to chamber at fireing ). Most common brass that is at .200 from the base, but take for example the 7 MM Dakota it is .130 ? the 6.5 Jap and 6.5 Carcano .150, alot of the belted mags at .219 / .220 but some at .250
I notice quite a difference in powder capacity between different brands of brass of the same caliber, due to the brass thickness and possibly due to how much of the case head is solid.
Back when I owned a 6.5 X 284 it was a wildcat that you made your own brass for, and remember a smith telling me that when chambering the 6 X 284, and the 6.5 X 284 he used the same ( 284 Win. ) headspace gauge for all of them.
I take these drawings for what they are, a general guideline but if you compare fired cases of the same cal. from different brand guns ( factory, custom, target rifles ect. they are all a little different because of the reamer used or headspace. Jed