Near the beginning, it was said that Hydrogen Cyanide was heavier than air. It is in fact lighter than air and although used in WWI, it had limited efficacy, as it dispersed in the atmosphere very quickly.
In addition, because HCN is highly flammable, using explosives to disperse it, would more than likely result in a fireball, rendering the gas useless.
In addition, because HCN is highly flammable, using explosives to disperse it, would more than likely result in a fireball, rendering the gas useless.