Dillon Border Shift Ammo Bag
An overview with pictures

This is a blue Dillon 'Border Shift' Ammo Bag. This is an ammo bag for the range with two sides. The Border Shift bag is designed to be used to carry lkoose ammo on one side of the bag, and the spent brass is placed in the other side when at the range. As you use the ammo, the side with brass fills up and the movable wall between the two sides shifts to allow room for the brass once the ammo is gone

As you use the ammo, the side with brass fills up and the movable wall between the two sides shifts to allow room for the brass once the ammo is gone