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I would be more inclined to go with the .243. You know.....you can load the big ones down, but you can't load the smaller ones up.
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Jim - My very thoughts when it comes to power. That's why I first considered the 6mm-06 60 grains of some slow powder should really get some 87 -100 grain bullet moving. But I've already got a 29 inch barrel 270 AI that can really move 100 grain bullets.
The idea of tiny goups on paper also appeals to me and it seems to me that on a national level it's only being done with 6mm XC and the smaller cases. Your limiting yourself to the 1-8 twist and only shooting 100+ grain bullets. I've not heard how 243's do with 1-10 twist lighter non match bullets and 30 inch barrels.
I don't really think that a little 6mm needs a muzzle brake
when it's got a big barrel. I don't have one on my 458's and they shoot just fine!