I had a 470 with a set of 360#2 barrels for awhile. I loved the 360#2 BUT the action was too wide for a proper fit and the rifle was heavier as a 360 than as a 470. Also a bit awkward in handling.

While a .375 or smaller caliber may sound enticing the practicality of actually building it would make it a heavy and unbalanced tool.

I think you would need to use a caliber that was similar in base dimension and thus keep the barrels at a reasonable weight.