This is a pic of the muzzles of a Kavanagh 12-bore Howdah Rifle that has been in my custody for many years,
showing the interesting ratchet rifling in use at the time.
The pic is borrowed from an article I compiled for our local Arms Collectors Journal, apologies for the low resolution b/w image.

The full article is online HERE for those interested in such things.
If time permits, I'll take a better photo and upload to replace this one.