Re: 338 lapua barrel length
You must reload the improved lapua. You can fire a standard lapua in an ackley improved chamber but no need if you are not reloading the improved after that. I wouldn't go with the ackley improved version anyway because there are some better improved versions out there for the lapua. If you do not reload I wouldn't even consider this round. You will be hard pressed to find a box of lapua's on a hunting trip in a pinch. Get a good improved version of the cartridge and learn to reload it. I just can't see building a rifle like your talking about and then being dependent on a very limited availability of ammunition that may or may not shoot well in your rifle. You will want to work up a good load for that rifle.
Predictions are difficult, especially when they involve the future