A 3 masted ship didn't carry a course on the mizzen, just topsail & topgallant. That space at the bottom of the mizzen had the spanker (gaff rigged). That makes only 8 yards. And if a 3 masted ship was going to give it the 'whole 9 yards', she'd have to fly the royals, skysails, and moonsails too! But that would make 17 yards??
My thought is that the ammunition belt makes more sense than anything else. But it would have to be a water cooled gun - certainly not an aircraft gun as you'd melt the barrel.