Here's a yarn about a .577 on a Greener action, but the same is feasible with rifled 12-bore barrels. Long-time posters here will have seen it before (several times!).

http://www.hotkey.net.au/~orrs/GreenerStory.htm

I have acquired a couple of other Empire Greeners to play with, and intend making a 12-bore double rifle with one, using a pair of E.R.Shaw tubes I imported from the States a while ago. I am procrastinating on this one though, since the only 'proper' way to get the weight and handling right will be to make a new heavy monobloc from scratch (probably a new forend as well!) Phew!!