I just went through a 6mm AI build, a 25-06 build and a new 257 Weatherby and like all of them. The 6 AI does come close to the Weatherby, 6x284 & 6-06, it may trail those by 50-75 fps maybe more. In 6mm you hit increasingly diminishing returns at the 6mm AI or 240 WBY capacities.
If you want to shoot 115s go faster than a 1-8 they shoot fine in my rifle but I am sure I am loosing some BC.
The 6-06 I think you could shoot it at 6AI velocities and maybe coax a little more barrel life out of it. Brass is easier to come by too.
Does my 6AI do anything I can't do with the 243s, don't think so except for ranges over 500 yrds or so, farther than I shoot at game, but it has more cool factor.
Standard 6mm is a better case design with longer barrel life than the 243 and usually 100-150 fps extra velocity.
