Forgive my ignorance but according to Wright the 440 gr and 480gr bullets should be launched at about the same speed, probably for regulation. If so, the heavier bullet would not have a much slower barrel time, but would have the extra weight.

This is the BP version of course, using 142 grains for both bullets. As an aside, can I assume the reason black powder kicks more is because it adds about 80 grains to the ejecta load than NFB?