I do not recognise it as any of the Polish coat of arms. Wondering if that may be signature of maker of cannon, possible German one? You mentioned it is trophy cannon - do You know when it was captured?
Похоже на шведский:http://en.wikipedia.org/wiki/Dalarna
So that would indicate it was in fact Swedish cannon, that's interesting.
There are several cannons with this coat of arms. I doubt that they are Swedish. I don't know about any Gunnery/cannon production in Dalecarlia. I assume that the arms had German origin.