How to characterize UE4 mannequin in Motionbuilder with IK and gun bones?

What’s the mapping template for characterizing UE4 mannequin in MotionBuilder to use with IK and gun bones in UE4 engine?
Do I need to somehow constraint IK and gun bones (possibly IK root bones too) to some bones from the arm chain? Or is there a method of characterizing all of the bones, twist, ik, gun bones included?