Maximum/best barrel length for .243

[rizla]

I am planning on getting a custom accuracy international AW .243 which will be used primarily for long range targets and also a bit of deer and goat hunting. I want to use 115gr DTACs and want the max possible velocity and accuracy for optimum range, but not so long that I start to lose velocity. I have heard that a twist rate of 1:7 or 1:7.5 is best..?
weight isn't really an issue so I want the barrel as good as possible. thanks