I would recommend, first and foremost, locating a copy of Bombards: Mons Meg and Her Sisters by Robert Smith and Ruth Rhynas Brown. It has a wealth of photographs and great text description, and more to the point, it includes several X-rays of the junctures between powder chamber and barrel.
You should also seek out The Artillery of the Dukes of Burgundy, 1363-1477 by Robert Smith and Kelly Devries. It's pretty academic and somewhat dry, but it has an amazing amount of information and a large section of photographs of surviving weapons (with multiple views and measurements!) toward the back. Because it relies on primary source information, it even has some interesting information on gun finishes and carriage construction.
I hope that helps! :grin: