This is a sure way to stir up debate. But its generally informative.
One must remember that all original rifles that saw very much use have been "freshed". This means they are bigger than when originally made. This is not supposition for the best reference to this see "British Flintlock Military Rifles" specifically the chapter on Indian Rifles in British Service (think its chapt. 6 or 7 book out in the shop) VERY interesting info on rifle use in America, how common freshing and rerifling were and other facts. For example by the 1740s the Shawnees and other tribes were well armed with rifles in addition to smoothbores.
The 44 and 50 are very common calibers in surviving flintlock rifles. However, there is a surviving early 1770s rifle pictured in the book "Steel Canvas" that is nearly mint and is .42 caliber. Note that at the time of the Revolution this rifle would likely have been considered too small for war by some (but look at the musket ball size of the time).
It was basically up to the person who used the rifle. Some liked smaller bores some liked larger bores just like today. However, bores over 50-54 were *probably* not the norm in early rifles from my readings and what investigation I have done.
BUT frankly we just don't know.
If you look at rifles from the Revolution that were little used you will find the bores are smaller than conventional wisdom has indicated.
See "Rifles of Colonial America" Vols I & II. Rifles #44, and 121 are certified RW rifles with comparatively little use and are 44 and 47 caliber. Rifle #96 is a later rifle, perhaps 1780s and in 42 caliber. Again its in very nice condition and was in England by about 1800.
There are other references to rifles under 50 caliber. One very good one in Huddleston's "Colonial Riflemen in the American Revolution" concerning a rifle of about 48 caliber.
HOWEVER. This is not meant to be a ultimate word on the caliber of rifles used in the 1750-1780 period. The reasons stated above, the fact that a lot more rifles have been used up or scrapped than survive means we have a very limited window to view American rifles of this period through.
If I were to make a typical Rev War rifle it would be 45-54 caliber depending on what I intended to use it for. From surviving rifles anything over 42 cannot be scoffed at. For shooting deer I would prefer a 50 over a 45, though the 45 will work great especially in the east. For elk I would rather have a 62.
To shoot most flintlock rifle matches and as part of a costume at a 1770s re-enactment a .45 is perfectly acceptable in a pre-Rev or Rev war rifle. Surviving rifles confirm this. With todays powder prices a 45 may be a plus. Just as it would have been 250 years ago.
Dan