PREMIUM-QUALITY CAST STEEL COMPETITION KETTLEBELL: Our hollow core kettlebells are made from a single cast, without fillers. Unlike welded and filled bells, they won't rattle or shift during exercise. EASY WEIGHT IDENTIFICATION: Our kettlebells is color-coded for easy identification and adhere to international competition standards for weight identification. The exterior dimensions of each size are identical. The different weight sizes are cast with varying hollow cores to attain the desired weight.