Pretty hard for anyone on this board to do, saw off barrels, but if "an old heavy duck gun" picked up cheaply at auction becomes the go-to for an active hunter as Marrakai by shortening the barrels it's not likely to be noticed---and the gun isn't retired. From the looks of the Greener, the barrels appear around 30 inches.
One of our most distinguished members told me on condition of anonymity that he always shoots cylinder/cylinder in field and range and has the trophies to show for their effectiveness. I did not ask if they were cut barrels. I wouldn't cut any of my guns but if I saw something to resurrect for useful work I'd do so.
Guns shouldn't fade away.
