They're unusual because they did not make many. Pretty sure you'll find that most French 16's are 28" (or metric equivalent) or less. I'd say you're about as likely to find a short 16, less than 26", likely made for woodcock with one barrel having dispersion rifling, than you are a French 16 with 30" tubes. I've owned a bunch of French 16's (need to count one of these days), never any that long--and I wouldn't have rejected one out of hand just because of long barrels.