Find an antique webley .577 revolverand make up some shot loads.
http://www.bing.com/images/search?q=...ey&FORM=HDRSC2
Those are some honking big revolvers. The best manstopper of the 19th century.
One might bore out a .500 S&W if the cylinder walls are thick enough. Probably have to have a cylinder built and the frame relieved to allow it to fit. The barrel should be large enough in diameter to bore out.
Short case 28 gauge brass shells would probably be needed.