Transitional rifle implies an orderly progression, and the old gospel said that Germanic rifles were short, thick and large bored, then in America folks learned they needed a longer barrel for a better sighting plane or to burn inferior powder or whatever. Some writers made stories up and they got passed along. But there are records of rifles ordered with barrels 4 feet long in the 1750's, so it's confusing at best. I think "early" is a better term.
However, the term transitional may apply as we think of architecture. If we see the Marshall rifle as Germanic in style, by comparison #42 looks lean and racy. Within a school or region, we see transitions. Over time, in general, buttplates got narrower, then curved, then deeply curved, etc.
It seems that in common use , the term "transitional rifle" is to describe a rifle with a combination of early European characteristics and characteristics found later on clearly American rifles.