Hello Seth,
You have good questions about these types of guns that interest many of us. There has certainly been an evolution in this country over time as has been discussed by OLUT (who has the largest, most complete, as well as heavily documented collection of percussion O/U American rifles that I know of). WonkyEye and Hungry Horse present good points, too.
I think the small caliber smooth rifle is under appreciated as a small game getter. As already mentioned, the large powder and shot loads they are capable of handing are more potent than modern 410 cartridge loads limited by chamber length. These smooth rife bores are also satisfactory for medium game when using patched round balls at relatively close ranges. They are potent back up loads when combined with rifle barrels. They were practical farm and homestead food providers.
Not yet mentioned in this thread is the fact that many combination guns of this period in both America and Europe had straight rifled barrels combined with twist rifled bores. Straight rifled barrels handled bird shot like the smooth bores, and also handled patched round balls a bit more accurately than smooth bores as indicated by Dillin in his early book on the Kentucky rifle.
My impression from studying many of these from the flint through percussion eras is that, in general, the smooth and straight rifled bores gradually enlarged over the decades and barrel lengths decreased. I think the interest in targeted species moved from mostly medium game and defense early on, to deer and small game combined, and then on to mostly small game in the late 19th century. This is conjecture and I have not tabulated the statistics on the 250+ Kentucky double rifles studied to date to objectively document those thoughts.
Thanks for bringing up this thread, and I hope others will put forth their ideas too.
Bill Paton, Kentucky double rifle student, Anchorage, Alaska 907-230-3600 (currently in “Eclipse Country” :-)