Some of you may have heard me mention that my great-grandfather had a punt-gun - a double-barreled four-gage muzzle-loading shotgun, intended for shooting flocks of ducks on the water.
I never saw it shot.
I just googled "punt gun, and discovered that four gauge was relatively small for these - that they commonly ran as high as .57 gauge - or 1-3/4 pounders.
And Tom Knapp has some video:
http://www.youtube.com/watch?v=n7FeeamC4qk