In the last installment, we talked about the growing need throughout the 20th Century to reduce the weight of the cartridge case, to lighten the burden of the soldier. Experiments in aluminum have thus far proven unsuccessful, but another material is even more promising: Polymer. Plastics and polymers burst onto the scene in the post-war era, and it didn’t take very long for engineersto start looking at them as a way to reduce the cost and weight of ammunition. If feasible, polymer is an ideal solution for cartridge cases, as it is even less dense than aluminum, while being cheaper and using no metals or other expensive strategic resources, just crude oil.
Polymer does present some interesting challenges to ammunition engineers, however. Common polymers like Nylon possess a high strength-to-weight ratio, even compared to brass, but their stength-to-volume ratio is poor. This means that polymer case walls must be dimensionally different than their metallic counterparts, and that traditional case heads with extractor rims must be made of a different material. This combination of a polymer case body adhered to a traditional metallic base is called a “composite” case design, and is perhaps the most mature lightweight case design today, with a few such offerings on the commercial market in high pressure rifle calibers (although these have had mixed success). This basic concept has been in development since at least the 1970s, but recently a company named MAC, LLC has developed a successful composite aluminum case, and in the process taken out several patents on their design.
The most obvious material to use for the base of a composite case is brass, but steel is also easily used, and either gives a modest weight savings of about 15-18%. However, I think the best possible result would be an aluminum base. Careful design of the polymer case body and metallic base could allow for a successful, reliable cartridge case that solves the problems of an aluminum cased round while giving a substantial weight benefit versus either brass or steel based composite ammunition, rivaling that of the CTA concept which will be discussed later.
Whatever material is used, the composite case concept is mature enough that it seems lightweight cased ammunition is just around the corner one way or another, whether that’s via this concept or something more ambitious. However, we shall save the latter for another episode.