Most "guns" on privateer craft were the 1" - 3" swivel guns, not really big.
Hah? You must be kidding! Swivel guns were few. How many ships have you ever seen w/more than 4 swivels, mounted either fore or aft? How could these guys accomplish anything w/o full-size naval/siege cannon bristling along the bow, under deck?
Sorry - not true. The range of ships used by privateers was quite large. They did not appear to include ships of the line, but did include vessels up to the size of navy sloops and cutters. Over 1700 letters o marque were issued in the revolutionary war. http://www.nps.gov/revwar/about_the_revolution/privateers.html
Cool. Makes me want to go out and get a more modern equivalent, the M20 75mm (3") recoilless rifle. After all, it's a "rifle" which makes it "arms" and not ordnance. IIRC, there's a 106mm rifle too.