Charles Bailey use to shoot a 6.5x57 improved in 1K LG. He did NOT start with the 7x57 brass -- the 7x57 has a longer neck than he other "_x57" cases. He did OK, but only that.
For what it's worth, the success of the 6PPC at short range has gotten everybody thinking about case dimensions -- chambering -- as a significant factor in accuracy. I think it is way down on the list. You're better off, I think, to pick a powder you want to use, and a bullet, Esp. a bullet.
Do I have a reason for thinking this way? Oh, brother. So many wildcat dies sitting on my shelf because at one time or another, I got the notion that "X" mattered in case design. And when the 6.5/.284 became popular, being a contrarian, I had to do something different -- a 6.5/06 AI. Joel Pendergraft got one too. For a couple of years, we cleaned up. So much for short and fat. Steve Shelp and Dave Tooley went with the 6.5x55 Swede improved. For a couple of years, they cleaned up. Then somebody with a hot barrel chambered in 6.5/284 would clean up. You get the picture.
I'd say you are much better off choosing a bullet, then choosing a powder. Use a ballistics program to find a case size that gives you 100% loading density with those choices. Then pick a case where you have to do the least amount of work to go shoot.
How to choose a powder? I dunno. Over the years, 4350 has seemed to work very well, just a little fast for the big cases. My gut feeling is the success of the .300 WSM is in large part due to the powders that work in it, 4350 being a prime choice. I've seen 4350 work well in the .300 Ackley sized cases, but of course, it isn't ideal, loading density is way down there. But I gush, always a warning sign.
Another thing I've noticed is that the double-base powders seem a bit more forgiving, a wider pressure range of good performance. But then, lot-to-lot variation seems greater. And the lot-to-lot burn rate, at least with powders from Alliant, seems to be set by blending granules of different sizes. Does that matter? I'm not sure.
These are just my prejudices. I've always felt that faster powders are more consistent performers. There is plenty of evidence to the contrary, *this* gun, *that* gun. I still feel that when you look at *all* guns, the faster powders seem to work more consistently.
FWIW, I'll go back to the first statement. Pick a powder, then a bullet, then a chambering. When picking a chambering, pick one that needs the least amount of work. Pushing shoulders back etc. gets old, and offers lots of opportunities to make small mistakes.
