Allen,
In the left column click english - click on 1st photo - scroll down to 'The material', and play the video (this vid has been posted here before) of the guy showing how brittle the cast iron is on a competitor's product (with the help of a small sledge and an anvil) - there's also a vid of a bunch of barrels being test fired. There are a lot of
interesting items that Herr Jan Schlottmann is manufacturing - take some time and skulk around this site.
Anyway, if you click on videos in the left column, then click on cannons, no.15 is the YouTube video of the 21mm breechloader that you have already posted, but I can't find any other info about this cannon on this site, although it's possible I missed something.
http://www.waffen-schlottmann.de/index_kanonen.htm
