If you're talking about the Midwest up to 1760s, it's most likely a French or Dutch trade gun. Certainly there were some Fusils de Tulle about, rifles weren't very common in the midwest in the early period. When the native Americans learned of rifles, they wanted them more than the smoothbore trade guns because they appreciated the value of accuracy. As the French & Indian War (we're talking the one from 1755-1760) was waged, more rifles were introduced into the midwest as settlers encroached upon lands traditionally hunted by the Indians.
As the Ohio Basin was settled, more rifles appeared and by 1775, it was boasted that in one county alone, over 6,000 riflemen could be raised.
Let's move on a bit. While New England gunsmiths could and did make rifles, the majority of guns made in New England were smoothbore fowlers or militia muskets. Rifle production was predominantly in Pennsylvania and Virginia where German settlers established the American rifle tradition. The short barrel, big bore jager rifles (go to Westpoint Museum for an example of a military jager rifle) evolved into the longer, smaller bore and slender American long rifle. Early examples were thicker at the wrist and the buttplates tended to be flatter. Because they were bigger bore, powder horns tended to be bigger during that period. By the time of the American Revolution, the rifles were thinner and most of them no longer had a patchbox with a sliding wood lid. Many now featured a brass one (remember there are always exceptions). Correspondingly with the smaller bore, smaller powder horns were used.
After the war, the guns became even more slender and the buttplates could become more crescent shaped (fired from the arm and not the shoulder). During the time when the Constitution was signed (1792), the "Federal" period came into being and the long rifle was reaching it's zenith in terms of architecture and refinement. While relief carving, engraving and wire inlay has been found on some earlier guns, they became more common during this time. You'll find a good example of a Golden Age flintlock at the Westpoint Military Museum.
Design and taste were regional. In the poorer counties, fancy architecture could be dispensed with & a working gun evolved. This is the origin of the Tennessee Poor Boy. Generally lacking in refinement, the poor boy features simple and clean lines and is a graceful rifle in and of itself. Most did not have a patchbox and had instead a grease hole cut into the stock from which the hunter could lubricate his patches. Iron hardware (trigger guard and buttplate) were common and the tang tended to be more spear point that the Virginia or Pennsylvania rifle. It was believed that the spear point would shatter the wood behind the tang less when the gun recoiled. Jerry Noble's books (Notes on the Southern Long Rifle, Vol I & II) are about the best you'll find on these guns.
Makers in affluent Pennsylvania and Virginia could afford to dress their guns up more and they did. There are styles (or schools) that distinguish guns made in one county to another. Customer demand often dictated how the gun appeared too (just like today the more $, the more fancy). James B. Whisker's series of books are excellent for photographic research. Ditto for George Shumway's Colonial Rifles of America and Joe Kindig's Thoughts on the Kentucky Rifle.
After Lewis & Clark returned home, several members of the Corps of Discovery returned West as professional trappers. Because a heavier gun was needed for the bigger game (think buffalo & bear), the bore became larger. Barrels were shortened to make the gun handier (and lighter because of the bigger bore). They were also become thicker so as to make them sturdier when carried on horseback. The Plains/Mountain Man Rifle thus came into being. Mind you, no single rifle is representative of this genre of Fur Trade rifle. The "Hawkins" itself was not the predominant arm that many are led to believe today. There were numerous makes but the characteristics generally follow the description given above (until someone who is more awake than I read this, spurt out their coffee from their nose when he laughs and types in a rebuttal).
The American Civil War is perhaps the most confusing period of American firearms history. The shortage of arms resulted in both North & South sending agents to Europe to procure as many arms as they could. Guns were purchased throughout Europe to meet the demands.
OK, let's stick to simple things. If you must have a rifle musket, get a Springfield (1861 or 1863) for either side. If the South didn't have it, it would capture it from the Yankees. If you want an import, the most commonly used imported rifle musket was the Enfield. It was also the most accurate of all rifle muskets. Both Springfield and Enfield come in various lenghts, but the long 39" barrel is generally the most common one seen in the pictures. If you want to play Confederate sharpshooter, go with the shorter, 33" barrel two band Enfield.
Enough for tonight. I'm tired and want to see the election results.
