Thank You Daryl
Forsyth and many other of the old school thought the "1 once rifle" was the best all around weapon for world wide hunting with the exceptions of Elephant, Hippo, Rhino and Cape Buffalo.
For all other large non-dangerous game, and all big cats, the 66 was the recommended size.
Many of those men killed more game in a month and any of us will kill in 20 years, so it think it's wise to listen to what they wrote.
I also am making a Rigby style 12 bore in the next year or so. The 12 bore is way more than is needed as you said, but loaded with a 1/2 charge it's no more than the 12 ga slugs that many folks use back east for deer hunting, with excellent accuracy too. If you load 200 grains of powder, it's suitable for everything, including careful picked shots at elephant.
I will hopefully have it finished in the next year or so. It's going to be a "speculation rifle" so I work on it as and when I can. My customers have to come first.