.240 Wby with 8 twist, hands down. Only reason I don't already have one is there aren't any long actions with .308 bolt face to be had. It would be worth using a factory 700 action and blueprinting. It's always on my short list of projects. The why is a rhetorical question: Have you ever hit a whitetail (or mulie) with a 100gr Sierra ProHunter with ~3400 ft/s muzzle velocity?? They change direction and invert spontaneously! Norma Weatherby brass is some of the best and most consistently concentric made. Almost worth throwing away your 25-06 and necking up the .240! Accuracy is just stupid even from a synthetic Howa. The only shortcoming of the cartridge is the twist rate availability, which is no fault of the cartridge. Theoretically, the 6x284 or even 6mm-06 would be faster potentially, but with the 240 there's no toiling with forming cases, annealing, etc. Just load and go. And yes, the 10 twist handles the 100gr ProHunter without issue, even at sea level, but I'd want the capability to go up to say a 108gr Elite Hunter. I think 7 or 7.5 may be too fast for 85 - 90gr cup and core bullets at those speeds, but the .240 seems to me to be about the perfect 6mm for dramatic killing results and EXCELLENT brass with super accuracy with no extra work.