Lancaster, with regards the 16 bore SXS you posted, 16 was the smallest bore size Forsyth recommended for dangerous game in India.

In Africa, 12 bore was deemed the minimum in muzzle loaders.

It was not until the advent of strong enough breechloaders that could shoot hardened balls (conicals) that slug shooting guns could compete with the bore size ML's hardened spherical balls for 'killing' power on the big beasts.