The buying public has already spoken on this subject it is the 243 Winchester, the most popular 6mm cartridge. It is the only 6mm cartridge to make the top 10 in sales of reloading dies. Given that status it is the ultimate.

ultimate - definition (dictionary.com)
maximum; decisive; conclusive: the ultimate authority; the ultimate weapon.

drover

223 Rem, my favorite cartridge - you can't argue with truckloads of dead PD's and gophers.

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