Quite a nice cannon & actually in pretty fair condition for its age & what it has been through. It is a British piece, with the Crown over GR (George Rex) & I believe there is the numeral 3, rather than 2, entwined in that cypher denoting King George the 3rd.
I think that the cannon is of the Armstrong-Frederick Pattern of 1760 (A.B. Caruana, Vol II, Chapter 4) - calibre is difficult to determine without measurements, I would suggest probably a 9pr & maybe even a 12pr. Thanks for posting.
Adrian.