I've hunted rabbits a bit with a 500/450 No2 Musket/14ga Westley Richards hammerless cape gun. It may "take some getting use to with the small & big barrels" but I never got used to it. For me, it always felt awful, muzze heavy, lopsided and clumsy.

It seems to me that if you want two very different barrels then an U/O is the way to go.

mike