A capsule is configured to have the outward appearance of a cartridge for a firearm and the capsule is designed to release a toxic agent when struck by the firing pin of the firearm. The capsule includes a casing; a bullet shaped container; a toxic agent, and a cup. The casing is made to fit within a firing chamber of the firearm. A bullet-shaped container holds the toxic agent under pressure. The container fits within the casing to give an outward appearance of an ordinary bullet in a regular cartridge for that firearm. The cup gives the outward appearance of a primer cup. Once hit by the firing pin of the firearm, the cup breaks, releasing the toxic agent. A primer and remote radio-frequency activator may also be used to release the toxic agent.