Heavy metal. The blackPack Steel Shots can flow smoothly during exercises when the blackPack is moved. The round surface of each steel ball makes for a smoother flow than any sand can offer. The Steel Shots have a diameter of 1.6 mm to 2.24 mm and are made in Germany. They are optimal for all blackPack sandbags. They are delivered in a strong plastic bag (dimensions: approx. 25 x 36 x 7 cm). Total weight: 25 kg (55 lb).