You will sometimes see, or hear, people refer to what makes cartridge casings work, which is their tendency to return to the same size, or close to it, after being fired as “ductility.” This is false. The tendency of a deformed material to return to its original size and shape is actually called “elasticity.” While it might seem we’re getting fairly pedanticat this point – it’s not like our cartridge casings are going to stop working if we use the wrong term – why not use the correct terminology? Also, an understanding of exactly what happens to the casing when we fire a gun can allow to us to better understand what happens, and why, when we handload.
Ductility is “the ability of a material to be drawn out longitudinally to a reduced section without fracturing under the action of a tensile force.” For instance, when drawing out lead into a long wire which can then be cut up and formed into bullets, that’s where ductility comes into play.
Elasticity is “the property of returning to an initial form or state following deformation.”This is why we refer to rubber bands, or the waistbands on underwear, as “elastic.” When they’re stretched then released, they return to their original size.
Similarly, when we fire a gun, pressure inside the cartridge casing causes it to swell until its expansion is stopped by the material of the surrounding steel chamber. Then, the property of elasticity in the brass used in cartridge casings in the Western world (which is present but less so in the mild steel used in cartridge casings in those “other” countries) causes it to rebound, not quite back to its original configuration, but instead somewhere between its beginning state and its point of maximum expansion. This means less friction as the casing is being extracted from the chamber.
In handloading, when we resize a casing, we are taking advantage of its property of elasticity to swage it back down to its original dimensions. In essence, we are “squishing” it back down. Which the elasticity of brass allows us to do.
All hail elasticity! It’s what allows reloading machines to work as well as they do.
