338 Fed is dying a slow uneventful death like all the other rounds that didn't catch the publics fancy .

This really is a no brainer , the bullet selection available in 30 cal is leaps and bounds better in 338 . Shooting to distance is all about BC and to get a good BC in 338 you got to go to a 250 grain projectile and the 338 Fed doesn't have the case capacity to take advantage of it .

This will be even more the case if this is an AR10 based rifle and being limited to mag length . AR10's are finicky bastards in 308 let alone other cartridges that it wasn't designed for . The further you stray from 308 and the powders associated with it the worse it gets .