The basics, if you import .c3d data in to MotionBuilder and then save your file, all takes will be set to the same end frame as the current take you were on when saving the file.
This is a pain but I created a workaround until it can be fixed by Autodesk.
Temporary workaround… Create a marker/junk attribute/anything that can be keyframed, and plot (with constant key reducer off) animation for all takes for just that marker.
Now save the file, your frame ranges are now saved correctly because key frames exist on each take at the proper length.
Brad