You mean like this?
my 6mm AI
7mm rem mag,6mm AI, 6mm rem, .223
6mm AI page
For your average shooter, I would suggest a .243, but for someone serious, I would start with 6mm and consider the 6x284 and .240 weatherby too. I like a slower twist with the 6mm and a fast twist for the 6-284 as it can better capatalize on the heavy bullets.