I think what you are refering too, is a gun pod, mounted externally to an airframe that when needed could be "ejected" or basically dropped if an emergency arises, as common sense would ask you to return with the pod to be reloaded again, and as it has the guns in it.
An ejection pod as I know it, is a part of an emergency egress system that ejects not only the seat, but usually the whole cockpit section to come down as a unit as opposed to a individual seat ejections. The only aircraft I know of with this system off the top of my head would be an FB-111.
