The MBS9 9mm laser bore sight adds the modular battery pack that allows you to remove the small watch batteries and end cap (included) and replace with it with an external battery pack that contains 2 AAA batteries (included) that will give you run times of 20+ hours for your laser bore sight. The external battery pack has a small magnet on the side to hold it to the receiver and allow you extra time to get your optics dialed in.