Great minds think alike (and so do ours!
I had the same impression when first looking at the pictures but the staff of the auction house assured me that the barrel actually consisted of one singular piece throughout. I think the caliber could be about 30 mm. The balls at that period of time were most probably made of stone; I got a few in my collection. That's where the special name of this kind of earliest guns came from: stone gonne (German Steinbüchse