Pappy I wouldn't fret pursuing a 6CM if you already had a 243 that shot well. That is assuming your 243 is able to stabilize the bullets that you intend to use while having a throat cut in such a way that you can reach the lands and still use your mag well. So if you're not diving into the 105 ++ high BC game then this isn't a factor what so ever.
Not fretting, just ruminating. I've only just barely begun with the .243 I have. With a 1-7.7" twist, it'll handle most bullets, except maybe the 115 Bergers. There's a place very nearby with a 300 yard range. Gonna join and see what this puppy will do. It won't be shooting across a fjord, but it's farther than I've ever shot at game. Gotta passable starter load for chucks; next job is a deer load with one of several suitable bullets on my bench: 90 AB, 90 ETip, 103 ELD-X, 95 BT, 100 NP, 85 NP, 100 SPH. Already used the 100 NP and Sierra in other rifles, so the pointy ones are going to be my focus.
