I love the .375 Flanged Magnum. It is a highly versatile cartridge that can be used on smaller big game, and is street legal for the big 5. Whether I'd want to use it on anything larger than leopard or lion is a question I haven't quite decided for myself. Would it work on buffalo? Sure it would. If the shot is well placed using a high quality premium bullet. My VERY limited experience suggests that things can and do go bad and lots of extra power is a very nice thing to have on tap. I'm absolutely uncomfortable using the .375 Flanged Magnum on elephant, or non brain shots on hippo.

The .375 Flanged Magnum is not the easiest cartridge/brass to find, so if your rifle is capable of handling a larger cartridge, it may be worth looking into a .450-400 which seems to be quite a bit easier to find with Hornady now producing ammo and brass for it. I'd prefer going to a .450NE if your existing rifle frame will accommodate it.

How your existing frame will balance with a heavier barrel set may be another consideration. It would be best to talk with a highly knowledgable double rifle builder and gunsmith to see what is possible and what is sensible. It may be more viable to simply get a used big double for big 5 hunting and enjoy your present rifle in its current caliber. I commissioned a double rifle with two barrel sets because I thought it would be easier to travel with, but financially, it would have been significantly cheaper to buy two good used doubles in a medium and big bore, and realistically, probably not much more inconvenient to travel with than a single rifle with two barrel sets. Is there another factor in Norway that would compel one to wish to have only a single rifle, such as gun control laws that make it difficult to own multiple rifles?