[taylorce1]

Do you reload? If you do, I'd start with figuring out how long of a cartridge you can fit in your magazine and feed. Once you figure that out, you'll know about what length of bullet you can use effectively in your action.

In an ideal world the base of your bullet will not be seated below the shoulder of the cartridge. This allows the bullet to be seated while not taking up powder capacity. You want to try to maximize both cartridge length and powder capacity to get the most from your rifle.

Once you figure out what bullets you can use, then you can decide on twist to maximize your options. There isn't any need to twist your rifle for 180 Berger Juggernaut bullets if all you can fit in rifle are 160 grain bullets. Standard twist for .280 AI is 1 in 9 inches, so a 1 in 8" would cover most bullet options well.

As far a length of barrel is concerned, the main thing is how will you use the rifle. If it will mainly be used at the range a longer and heavier barrel may be the best option. If it will mainly be a hunting rifle then weight and balance are more important so you might need a shorter and lighter contour barrel. So figure out if you want mainly a long range target rifle or a hunting rifle first. Then build the rifle accordingly to its main purpose.