It seems that there is not enough difference between the two (shooting the same bullet) to matter. If anything, the .243 has slightly more case capacity. Is it a matter of barrel twist rate for the heavies?

Yes, the 243 has slightly more case volume than the 6mm CM. There's not much difference. The 243 can push the same bullet slightly faster by about 50 to 100 fps. The 6mm CM is generally spec'd with a faster twist to account for the heavier bullets. Most custom 243's run a similar twist to run the heavies.

Creedmoor case design is more efficient.

260 Rem and 6.5 Creedmoor are the same way. The .260 requires more powder than the 6.5 Creedmoor to get the same bullet going the same speed, on the same length barrel.

And improved case geometry often makes for more consistency shot to shot, lowering ES.

Look at brass length of both as well. And you only have so much room to play with inside the magazine.

Never shot or loaded for a 6mm CM but for me I would buy a 243 win twisted to what you want to shoot through it. Any advantages of the CM don't out way the common and available 243 brass ammo etc.

Can a 243 be had in the faster twist without a re-barrel?

Good question Gary. I cant think of one off the top of my head, but I feel like I remember seeing some before. I was thinking strictly in the aftermarket though.

Savage is a 1-9 twist. I shoot 105 amax thru mine. Ruger Precision gen 1 is a 1-7.7 twist. Hard to find those.

I'm waiting for the 7mm Creedmoor since the case is so efficient. Maybe even a 308 Creedmoor. I bet that would sell like crazy.

trolls are out early today

Case capacity is too small for 7mm and .308.

It can be a razor's edge of works, and doesn't work.

LOL. Just using logic. Everything Creedmoor is better than everything 308.

.284 WIN cartridge is my favorite, it doesn't get enough love.