Those are old milkcans, what farmes used in the past century (till the 1960's or something for regular use, some farmers use them only by their own farm now).
What you see is a old shirt or sheet to reconize the top of the milkcan and to limited the distance. They made a little hole in the bottom, put carbid (don't know if this also a english word) and a little bit water in it. Put the top on the can, lay it down and sit on it. Fire by the hole and you see the result. It is not importend to shoot the top far away, but to make a loud noise. Sometimes they put a soccerball in the top, see one of the new clips.
here some more clips, with city impression. Only town where it is legal to shoot on streets in these period!!!
http://www.youtube.com/watch?v=uHYq60b_ ... re=related
http://www.youtube.com/watch?v=y4R4GuN4mDc
http://www.youtube.com/watch?v=88dAEpN9DH4