Hi all, 
thanks for the answers.  I tried to post last night, but it doesn't seem to have come through.  I'll try and get some pics when I go up to se my uncle for christmas, but I make no promises, as I have too many hobbies, and not enough memory or time some days.
The cannon that he has has a hinged breech block that opens in line with the barrel.  has a pin that holds it closed IIRC.  I think he had someone turn a small bushing to go into the breach to change it from 8 ga to 10 ga, as he was having trouble finding 8 ga stuff here.  am I correct that 8 ga shotguns are no longer permitted in the US?  which would explain the difficulty in finding shells.
I tried to post a film of this little bugger going off, but those type of attachments are not uploadable here.
-jon