Hi Fernando K,
Thank you very much for the blunderbuss pictures. They are good enough to see the mechanism.
You are absolutly right Michael, if it wheren't for the high cost they would have made there cannons of brass more often as well (some swiffel guns where made of brass).
edit: the blunderbuss from Twigg looks real enough to me. There are 4 twigg's and one Twigg & Bass in Der Neue Stockel on page 1307 (book2). The blunderbuss in the pictures are most likely of John Twigg (1732-1790) i think in Piccadili,London Great brittain where he worked from 1776-1788.