A weighted body adapted for insertion into an existing gun magazine shell approximates the weight and/or density of live ammunition cartridges. The size and shape of the body allows for insertion down and into the slot at the top on the magazine, pressing down on the spring, with a sliding retainer stop or other feature to maintain the weight in position, filling or partially filling the internal volume of the magazine with the weight. The integral slide retainer stop attached at the top of the device is then moved or slid by the operator into a position that locks the weight into the magazine so it cannot pop back out due to spring pressure and will not interfere with other workings of the gun slide or receiver. A plurality of color-coded stops may be provided for use with the same or different weighted bodies corresponding to different calibers of ammunition.