I have two rifles that are 28" Bull barrels with brakes on them.One is in 6.5X284 the other in .243 both 1-8" twist. The 243 is a laser, hitting 8" circle at 600yds in matches first shot ever time out. When I rebarrel it will be in a varmit contour with a brake in 26" or 28" the extra barrel lenght will be better for velocity and won't hurt much for unsupported shots, most shots are supported some way.Any of these chamberings will work well. You might want to also think about doing something in 6mm, there is the .243, 6XC and 6X47 that shoot very good with 105gn and heavier bullets when shot thru a 1-8" twist or faster. Plus have less recoil than the 6.5 guns.