RE: ? 6mm improved
I have, in the past, owned both the 6MM Rem AI and the .243 AI. Both are real nice calibers, and produce around 175 fps more velocity than the parent case. Brass life can be very good as well, with no need to ever trim cases.
But if you are looking for a high performance 6MM, the best of the lot is the 6MM-284 - in my opinion.It is a little better caliber than the .240 Weatherby; and now that Norma and Hornady make brass for 6.5-284, there is a readily available supply of high quality cases that require minimal work to neck down. The old .284 Winchester cases worked fine, but they needed to be necked in at least two steps and are not as consistent as the Norma cases.