there is some stuff about punt guns on youtube

http://www.youtube.com/watch?v=O9r_ZckAmkc

http://www.youtube.com/watch?v=EzwbcVbE9rw&feature=related

http://www.youtube.com/watch?v=fg85k28b_dM&feature=relmfu

http://www.youtube.com/watch?v=fg85k28b_dM&feature=relmfu

http://www.youtube.com/watch?v=IEAIQyVvINU&feature=related

I had once a collections of punt gun pics. a lot of different guns but loose it in a computer crash.

punt guns and svivel guns(same kind of big shotguns but used on a pivot in duck hunting blinds) were used also in continental europe. I know it about france, switzerland and italy till know.
in french they are called Canardieres and the bore of the french guns have seen is given as 37mm so I believe its the old 1,5" eley punt gun cartridge. beside with the french italien 34mm punt gun cartridge the only factory made ammo for this kind of guns. the eley 1,5" case is the same as the 37mm flare gun case. the italien 34mm case was used also in switzerland for a 34 mm signal flare pistol.
this is a 37mm punt gun made in saint etienne and used with 5mm shot on the lake neuchatel in switzerland up to 1962 when this kind of hunting was forbidden there.
265 cm oal, 226 cm barrel length, 80 kg weigth