You know the "Superball" that kids play with? Extremely elastic - throw it against the ground, it'll bounce fifty feet into the air? Hard rubber, small, heavy for its size? Put one in a shotgun with a magnum powder load, and fire it at close range. Voila - "stingball." Knock their ass back twenty feet, leave a baseball sized welt. There also exist "stingball grenades" for, say, prison riots.