One thing to consider is that as the bore diameter increases (larger calibers), there tends to be less benefit from a longer barrel.
I think this is because as the bullet moves down the barrel, the larger caliber means that the volume of bore behind the bullet increases much faster than it would with a smaller caliber. The result is that the expanding gases have to fill a much larger area and that reduces the pressure much more rapidly as the bullet moves down the bore. Less pressure means that the bullet isn't accelerated as rapidly as it normally would be--at least after the first few inches of barrel length.
Obviously, you can just design the cartridge to hold a lot more powder to compensate for this effect (as with the .50 BMG) but if you stay in the same general neighborhood in terms of power, expect to see less benefit from a longer barrel with larger bore diameters.
Sorta goes along with the results of Art's posted example.