Daryl is correct. The 66 cal (16 bore) English was the “high water mark” of hunting rifle development within the British empire in the era of 1860 to 1880. In fact, the acceptance of the breach loader is the only step forward which caused these kids of rifles to pass into obsolescence. There were equals in use from other countries, but the English stalking rifle was the epitome of for and function of the muzzle loading era.

There days, the 62 cal is more popular with America hunters then the 66. The reason for that is that the Brits hunting in India and Africa as their common ground and Americans usually hunt in their own country. In the USA big bears and bison are about the only 2 animals that you might worry about in the rare instance that they decide to fight after you pull a trigger. But those 2 animals are easily killed with a 62 cal. If you like 66, that’s fine. I build English guns up to and including 2 bore (slightly smaller than a golf ball)
Anyway, English stalking rifles are pure joy and they are as wonderful to hunt with today as they were in the 1860s.
Here are some pics
There first 4 are of a Rigby Style 66 cal.

Here is a Purdey stly 62 cal.

There nest 2 are of a Purdey style 8 bore