Creations, not sure what is causing the problem (has the gun been scaled?), but try this, instead of parenting the left hand to the gun parent the left hand to the right hand, as the gun moves with the right hand, parenting the left hand to the right should be visually equivalent to parenting it to the gun, and may avoid the problem. Replicant, parenting a hand will not work unless IK is turned on for that arm.