After importing a prop, try to give it a figure parent (head, rHand ...) as soon as possible. Then pose the figure and the prop will follow. Example: If you first pose a figure into a shotgun-holding pose and then try to move/rotate an imported shotgun into its hands, you will probably run into this problem. Trick is to import the shotgun first and then to make the figure conveniantly touch the shotgun e.g. The hand "graps" the barrel. At this time parent the shotgun to the grabbing hand. The shotgun will now follow the movements of the hand and posing the prop will be a lot easier.