RogueWars I am attempting to control the firearm's fire-primary bone to aim at the mouse's location.
It this really what you want? It sounds a bit weird, as if you're trying to point the muzzle flash towards the target while leaving the weapon aiming in another direction. Are you sure that you don't want to point the hand holding the gun towards the target? holder-primary would be attached to the same hand bone then. You could then take the Spineboy example scene as a reference on how to setup the complete aim-IK setup.
RogueWars Upon modify the weapon's fire-primary bone's Rotation, nothing happens; I suspect this is caused by the weapon's root being overwritten by the player's holder-primary bone.
You would need to share more details about your setup, what bones are overridden, and how you do that. In general a child bone can still be pointing anywhere through a SkeletonUtilityBone in mode override, I don't see why it should be overridden by setting the parent.
