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Why the change, 6mm 6mm 6mm now jumping up to 6.5??
Just an itch for something differance or a feeling that you want a larger margin of error for novice shooters (while still easy to shoot) and...

Do a custom build 243 with a 8 twist or faster barrel from a local manufacturer.
I run 108 gr ELD-M's in mine with great effect.
You can run more velocity with a 243 having a slightly larger case...