and increase the chance of achieving the direct hit necessary to kill armor. Many modern armies make significant use of cluster munitions in their artillery batteries. Artillery-scattered mines allow for the quick deployment of minefields into the path of the enemy without placing engineering units at risk, but artillery delivery may lead to an irregular and unpredictable minefield with more unexploded ordnance than if mines were individually placed. Signatories of the Convention on Cluster Munitions have accepted restrictions on the use of cluster munitions, including artillery shells: the treaty requires that a weapon so defined must contain nine or fewer submunitions, which must each weigh more than 4 kilograms, be capable of detecting and engaging a single target, and contain electronic self-destruct and self-deactivation systems. Submunitions which weigh 20 kg or more are not restricted. Chemical Chemical shells contain just a small explosive charge to burst the shell, and a larger quantity of a chemical agent or riot control agent of some kind, in either liquid, gas or powdered form. In some cases such as the M687 Sarin gas shell, the payload is stored as two precursor chemicals which are mixed after the shell is fired. Some examples designed to deliver powdered chemical agents, such as the M110 155mm Cartridge, were later repurposed as smoke/incendiary rounds containing powdered white phosphorus. Chemical shells were most commonly employed during the First World War. Use of chemical agents of all kinds has been forbidden by numerous international treaties starting with the 1925 Geneva Protocol (not to be confused with the Geneva Convention), with the 1993 Chemical Weapons Convention being the most modern treaty which also outlaws production, stockpiling and transfer of such weapons. All signatories have renounced the use of both lethal chemical agents and incapacitating agents in warfare. Nuclear artillery Nuclear artillery shells are used to provide battlefield scale nuclear weapons for tactical use. These range from the relatively small 155 mm shell to the 406 mm shell used by heavy battleship cannon and shore defense units equipped with the same guns. Non-lethal shells Not all shells are designed to kill or destroy. The following types are designed to achieve particular non-lethal effects. They are not completely harmless: smoke and illumination shells can accidentally start fires, and impact by the discarded carrier of all three types can wound or kill personnel, or cause minor damage to property. Smoke Smoke shells are used