While SteelBell creator Hyperwear also makes neoprene SandBells, these steel shot-filled versions of the free weights are about half the size of and, some say, twice as dynamic as their counterparts. Available as individual weights ranging in size from 5 pounds to 125 pounds (Amazon appears to stock a 10- to 40-pound range), SteelBells add training intensity, build core strength through constant stabilization, and reduce injuries (plus floor damage) related to throwing/dropping weights during workouts.
Within SteelBells' extra-thick neoprene casing lies a horde of steel shots. During movement the shots dart and shift around the bag faster than sand, thereby increasing SteelBell instability and forcing users to grip harder, beef up focus on balance and coordination, and purposefully brace core muscles to control the "live" weight.
Another way to look at it: SteelBells' shots move more like water than sand. By comparison to SandBells, or a traditional free weight, a 20-pound SteelBell will feel and respond like a much heavier weight. SteelBell retailer Rogue Fitness describes it as "the difference between picking up a sleeping child and one that is flailing."
In terms of size, the diameter of a 10-pound SteelBell is 9". A SandBell of the same diameter weighs just 4 pounds.