This very meticious article (in German) also concerns the Milemete guns. Although my German is only limited, I found it to be very interesting.
Cool page about the earliest cannons
I came across this page when surfing: The Milemete Guns. It contains an analysis of the pot-de-fers known from Walter de Milemete’s De Nobilitatibus, Sapientis, et Prudentia Regum. It’s well worth a read if you are into handgonnes like us!