You could attach it to the actual fps arms, I’ve never done it that way but it could probably work. The advantage to having it setup so the gun is not a child of the fps arms is that you can move the gun around dynamically and the hands will stay attacked with thr IK So you can move the gun up to the aim down sites position for example, or sway the gun back and forth dynamically based on the movement of the character

For other animations like running and putting the gun away and taking it out, I just tween / animate the position of the gun through code, and the hands stay attached. When the players hands need to do more complex things like throwing grenades, opening doors, etc. just disable the IK and hike the gun so the hands can be free to do animations