Wow, I've been "Linked"! :grin:
While you WILL find actual 18th century examples of what would be considered "barn guns", most of what people would call barn guns are from the early to mid 19th century. One gun I saw at a show was of "ordinary" stock design and construction, and was built with an old musket barrel and a 1760's English lock. The English type triggerguard was missing. It once had a sliding wood box lid, now lost. There was NO stain on the plain maple stock, and the wood was a nice yellow from years of handling/wax/dirt. This could have been built in 1770, or it could have been built in 1820...hard to say.
The nexus for 19th century barn gun production seems to be in eastern Berks county, PA. There are actually still in existence quite a few of these guns from Berks and Lehigh counties. I have gotten to see and handle a couple of them, and have photos of several more. I prefer to use the term "farmer's gun"...
They are built normally, but without buttplate or sideplate and only one or two upper rod pipes. A simple strap iron or brass guard is often used, though you will find examples with normal brass guards. They do seem to be often built a bit "oversize"....NOT "fat" but "fully figured".
The ca 1810 Berks county "Bauernwehr" that is in my photo album was modeled on the two original guns I have gotten to see. I really like doing guns like this, and actually hated to sell that one. They make great "beater" guns. I am EXTREMELY clumsy, and a fancy gun I will beat up pretty quickly. These are kind of like the old surplus Mauser rifles...you don't worry about banging up the stock a bit.
Southern "poor boy" rifles are something else entirely, and someone with more knowledge than me can tell you more about them.