Things to consider is the primary purpose for the arm. Some of our ideas or theories are skewed by romantic notions and intrusions of our reality and the reality of other periods and places. For instance 1840s Appalachia is not really the same place as 1770s Appalachia. Geographically yes but for everything else...not so much.
Since the OP asked about early Lancasterian rifles lets consider the time, purpose , place and for whom they were built.
Time 1763-1783. Most scholars agree this era was the period in which the American Longrifle and the American Rifleman emerged. Very very few American rifles are positively dated prior to 1770. There is period documentation of these early rifles but very few surviving examples. Examples show that early rifles tend to be of larger bore.
Purpose, primarily hunting and protection. Hunting for what? Deer sized game, this was the height of the deerskin trade. Protection from what? Men and large predators. It is unwise to be under armed. If your adversary is "rifle armed" it's best you be "rifle armed" as well. This goes for competition in hunting thus acquiring your livelihood and for the more important and dire contest as well. Again it's unwise to be under armed. Bore size of mid 40s and up tend to be more effective in these types of contests and most rifles of this period reflect that.
The place for the American longrifle in this early period was the hinterlands, the frontiers of the colonies, the back country as you approach the Appalachians, the mountains themselves and the country immediately beyond. It was down the Great Wagon Road starting at Lancaster, through Emmittsburg MD,Winchester VA, down the Shenandoah to Southwest VA and eventually the area that became western NC and TN. As I have studied my family history I see the same place names as in my study of the history of the American longrifle.
Now for whom they were built. It is my opinion that the rifle of this period was built for middle or middling class men. Those used it to work for a living either as a hunting tool or as a weapon, likely both.
The masters of the "longhunt" were the Indians, I dare say the longrifle was as much an Indian arm in this period and afterward as the bow and arrow. The rifle was very prized amongst the Indians, so much so that rifles were part payment of land deals and the British copied Lancaster rifles as items of trade to keep their Indian allies.
What does all this have to with caliber size? Quite simple, since the rifle of this period 1763-1783, was tool of substance and survival, caliber size reflects that. Too small there is a loss of effectiveness and range. Too large, there is a loss of usability and efficiency. While there is always exceptions, most rifles of this period have bores from the mid .40s to mid .50s. The first US rifle, the 1792 Contract had a specified bore size of around .49. Personally I think that is typical and an all around good size for the frontier rifle.
