Zulu, You are correct, Sutter did have one in Sacramento, CA at his Fort. We went back in time to that fort, just as the last group of Russians was leaving to get these photos of the 3-pdr. Unicorn Cannon for you.
The wheel diameter is about 10 inches less than a full-size CW 12-pdr Napoleon wheel, about 47 or 48". You can interpolate the rest of the dimensions from this size. Maybe the Fort has drawings of this gun, but we doubt it. They were more into grinding grain, medicine and dry goods displays. We did spot three or four fired aluminum foil powder cartridge remnants, so at least they fire their cannon occasionally! All in all a nice place to visit if you are ever near the town from which the 'Governator' governs.
Nice work, Zulu! Very nice.
Tracy and Mike