Have a look at this link and see the full discussion
but Marrakai points out something interesting

"I should add that the Greener 'Empire' is a massive shotgun with a long action-bar. The Brits distinguished fairly well between 'game-guns' and their 'pigeon-guns' or 'fowlers'. The latter two are usually heavily built, long-chambered, and proved for heavier loads. Many early pigeon-guns even had 3-inch chambers. This distinction is not always clear-cut in Continental or American guns.

Furthermore, a 12-bore Greener 'Empire' is about 3mm wider across the breech-face than the 10-bore Zabala! True! Might pay to put the calipers across that Centaure before making a decision, though the one on GB sure looks wider! I'll measure my Greener's chamber-wall thickness on the weekend and report back. I have three Empires BTW, two of which are still 12-bore shotguns! "

http://forums.nitroexpress.com/showflat.php?Cat=0&Number=39054&page=3&fpart=4&vc=1

and this which is the 577 built on a Greener.
http://www.hotkey.net.au/~orrs/GreenerStory.htm

It shows the long action / water table of the Greener
very well.