This 15 lb saddle bag-design Shot Bag from Kupo is filled with 2mm-sized steel balls and is used to stabilize a stand by draping it over the legs and preventing a top-heavy light stand from falling over, while it is also used as a counterweight on a boom arm to balance a light fixture's weight.
The bag is made from a black Cordex textured nylon, and the underside has a polyurethane coating which makes it water resistant and durable.
