First World War ammunition was typically composed of a lead core covered in a copper jacket. Lead is used because it is both dense, reducing the effects of climatic conditions such as wind (at least to a certain extent) and because it is relatively affordable and cheap to mould in large quantities. The copper jacket is used to reduce the effects of the firing mechanism on the shape of the bullet. The creation of hot gases by the ignition of the propellant can deform lead bullets, the copper casing prevents this, preserving the aerodynamics of the bullet itself.