It is your call, but if you're going to hunt deer and elk with the rifle, the Speer on line manual gives the 7mm O8 at 175 grain bullet with a velocity of about 2620fps max. The .308 Winchester will get 2690 max with 180 grain bullets. Both these figures are from 24 inch barrels. Also, the action is designed for the .308 length case. The 257 Roberts is a 57mm case and you might need a little to a lot of action work to make it feed reliably. Also, I kind of like Handskill's suggestion. You could go up to 338 Federal or 358 Winchester. Both will push a 225 grain bullet at around 2550 fps from a 24 inch barrel, the cartridge dimensions are exactly the same a the .308, and both will kill anything up to elk out to 400 yards with a 200 to 225 grain bullet. I wish I had thought about it first. A 215 grain Sierra can be pushed to 2670 fps in the .338 Federal, and has good sectional density and a reasonable BC. This would absolutely flatten a Whitetail or Mule deer and do respectably for elk. Either caliber would work good on bear, too, with the heavy bullets. I know the .35 Whelen loaded with the Speer 250 grain or the Sierra 225 grain bullet will spin a Mule deer or a Whitetail completely off of its feet at 400 yards. Energy for good hand loads in the Whelen is in the neighborhood of 3,500 to 3,900 ftlbs. With the .358 225 grain bullet at 2550 fps, you'd get around 3200ftlbs at the muzzle. You'd get the same with the .338, but with a BC of .498, and a trajectory similar to a .308 with 180 grain bullets. You'd have 2,000 ftlbs of energy at 400 yards. That makes this a good elk rifle at 300 yards and a good moose rifle at 250yards. Just something to think about.