The transitional period, in my mind, is when rifles with strong European styling associations were still being made here. Barrel length is often a focus, perhaps because so many Germanic rifles of the mid to late 1700s had very short barrels, and few such guns, if any, rifles with very short barrels were built from scratch here with new parts. But longrifles with barrels in the 36-40” length were being made in Europe during this time. Longer-barreled flintlock rifles were not a colonial American invention or enhancement, but an available choice.
If you have RCA volume 1, look at 17, 18, 19, 20-22, 40-43, 47-49, 51 and 52 for starters.
The 2 Moravian longrifle books are great resources.
Online look up the Deschler rifle, tulip rifle, Natty Bumpo rifle.
Much more in RCA volume 2 and other books.