You'll have better luck looking at antique German rifles - liner aside - because they were not used to the extent that American rifles were used. There are not many antique American rifles of the 18th century that are in clean enough condition to look at something like a vent diameter. RCA42 might be one to look at, the 'ghost' Peter Resor rifle is another, the Reedy mentioned in another thread that will be coming up at auction, possibly the 1787 Peter Neihart... there are some out there but not many.
By the time you get in to the 19th century you are generally looking at smaller bores and smaller barrels. This all has an affect on things - the bore size, the size of the barrel breech, where there is internal coning or notching of the breech (most of them) etc.
I've had a lot of very nice German rifles through here, many with gold liners, and the average size I would say was between 1/16 and 5/64.
For rifles without liners, even coned on the inside, I still like to go a little bigger than 1/16 provided it's at least 45 to 50 cal.
