As said you cannot use absolutes. At times I hate the term Southern Mountain and especially dislike "Po Boy/ Poor Boy". Now I use SMR quite a bit but It is almost as ambiguous a term as longrifle or Kentucky. Especially if you consider Southern Mountain can refer to any gun made/used? in that region from 1775 to 1935.
Are rifles and guns from the Carolina Piedmont region SMRs? Are Georgia rifles and guns like the Higgons Gamecock a Southern mountain? These may be Southern but not necessarily Southern Mountain. See what I mean.
You have to be more specific in region and especially time. What most mean when they say Southern Mountains are iron rifles made in East Tenn, Western NC, Northern GA & AL, West Virginia, Southwestern Virginia and Eastern Kentucky from 1800 to 1900. Wow!! That's a lot of geography and time there.
Below is a "Southern?" smoothbore that might and I stress might have been a original halfstock.
http://americanlongrifles.org/forum/index.php?topic=14829.0 There are a few purpose built smoothbores from that region, most show rifle attributes, IE smooth rifles or buck and ball guns. Most have octagonal bbls. A few have Oct to Round and fewer still have full round.
A good many of the smooth rifles may have been older large bore rifles bored smooth. There is a debate about this with collectors but most agree that some smooth rifles were purpose built.
By far most were fullstock. In fact I do not know of a confirmed round barreled "mountain" made halfstock smoothbore. There may be one, but in my studies, I have not seen or read about one.
The closest I have come to that is a old French cartridge conversion musket converted to a shotgun attributed Mathew Gillespie in NC.
Here are some accepted facts that maybe will help you piece this together.
Smoothbores by far were the most common arm from first contact until 1763.
Coastal areas from New England to Savannah tend to be Smoothbore territory in the Colonial era.
IMHO the early Dutch, French and English Trade guns in Indian use, especially with a rear sight, are the direct ancestor of the American long rifle. Maybe not by design but most definitely in use.
By mid 18th Century the deerskin trade is a major economic factor for the Indians and the Colonies. Today "Buck still means Dollar.
Rifle barrels are in import lists from cities like Boston and Charles Town (Charleston SC) as early as the 1720s. No confirmed American rifles this early are known but one would assume they would look like European pieces.
Legend says Boone's first rifle was a English Short rifle , late 1740s?
Germanic Moravian and German gunsmiths start producing rifles in NC and Pennsylvania. There is debate who was first.
The rifle becomes the favored frontier arm by the time of the Revolution. This very well could be due to Indian use due to their culture and the deerskin trade.
Rifles schools begin to emerge from Lancaster PA down the Great Wagon Road to SW Virginia.
Southern Colonial Rifles tend to be more Anglo then Penn made rifles that tend to be more Germanic.
Rifle regiments from Penn/Virginia frontiers tend to dress like Indians and favor Indian tactics.
Since rifles are a frontier arm the "Rifle Culture" starts to emerge in these frontier areas.
Due to the British fear of the frontier riflemen, Washington states that the Rifle Shirt would be the perfect uniform for the infant Continental Army. Part of this could simply be that it was cheap, easy to make and intimidating.
Riflemen prove to be undisciplined, vulnerable and of little effect in linear tactics. Riflemen do have great success by being supported by light infantry (Saratoga) or by using Indian tactics (Kings Mountain).
After the Revolution the "Golden Age" begins for the American Rifle.
By 1800, while rifles made in the East are more ornate. In the West more and more gunsmiths/farmer gunsmiths begin to use native iron as furniture for their less embellished plain rifles.
While halfstock guns have been around for centuries, halfstock rifles and smoothbores come into fashion in Europe in the last quarter of the 18th Century. The M 1803 rifle shows English Sporting rifle influence.
Gunsmiths are sent to the Southwestern tribes in the plan for civilization. Wiley Higgons is one of these.
In 1805 The trade factor at Fort Soddart (Mississippi Territory) reports "Smoothbores are unsaleable to the Choctaw, they want Rifles".
England is still importing smoothbore trade guns into Canada and a few into their trading factors in Spanish Florida.
Sam Dale a Mississippi Territory scout, is said to have used a double smoothbore or double musket in the 1813 battle of Burnt Corn. Most likely this was simply a early European made Double barreled shot gun. An odd gun for the that time and place.
Excavations From Fort Mimms 1813, reveal that the defenders and attackers were using rifles most around .45 caliber or smoothbores and muskets armed with buck shot. Hardly any musket size ball were found.
In the 1820s an Englishman traveling through Kentucky notes the lack of smoothbored guns.
In the late flint, early percussion era you will see a few halfstock eastern longrifles. Many of these will have wooden under ribs. Onto the percussion era you will see some Southern Mountain (Tennessee Rifles) with 3/4 stocks.
An ancient Carolina trade gun was found with militia markings and was converted to percussion in Memphis Tn.
http://americanlongrifles.org/forum/index.php?topic=6257.0 In closing a half stock smooth bore made in the Southern Mountains IMHO would be a phantom gun. A phantom gun is a piece that could have existed and a modern builder uses research and a best guess to create or recreate one. These type of projects can be great fun.
Dr. Tim Boone built a Wiley Higgons based Georgia Fowler a few years ago.
http://americanlongrifles.org/forum/index.php?topic=19799.0