Agree with 500Nitro: .300 Flanged is a marvellous cartridge for a double, and very much under-rated in today's "new-is-better" society.

Another marvellous light DR cartridge is the .32-40. It looks just like a perfectly scaled-down .450 NE, and was loaded by Kynoch for decades. With a 165gr bullet at 1750 fps it is a joy in a light rifle, and was very popular for the smaller deer and antelope. Its a personal favourite of mine, and I'll own a double so-chambered one day although for now I have to be content with a vintage Martini single-shot.