OK, folks,
I realize it's my turn to present the real stuff.
Please see Wikipedia:
http://en.wikipedia.org/wiki/Leather_cannon
Attached please find pics and description of a so called 'leather' cannon, ca. 1620's, preserved in the Historisches Museum Berlin.
As David pointed out the barrel actually is of copper reinforced by iron, covered with strong (elk?) leather and bound with hemp, so it's not just all leather ...
I learned they have two pieces, their lengths being ca. 2 m and the caliber a one pounder and the other a one-and-a-half pounder.
Best,
Michael