Ever wonder how a centerfire brass cartridge case is made? I didn’t think so, but since you’re a gun nut and I’ve asked the question, you’re now probably wondering.
Mechanically, it’s pretty simple–so long as you keep things clean—because just like with handloading, dirt and oxidizers can damage the dies. Metallurgically there are important steps throughout the process you have to take or you end up with brass that is too hard or too soft, or not hard and not soft in the right places.
Brass cases typically start out as a flat band of brass. Discs are punched from that band and obviously the bigger the final case, the bigger the disc you start with. The disc is formed into a small cup using a punch and then run through a series of progressive dies to lengthen it and thin the sidewalls.
Each time the case is run into a die, the brass is “worked,” and brass hardens when it’s worked. To keep the brass from getting brittle, it’s annealed by heating between each die. That heating also causes the brass to pick up rough oxidizers on its surface that have to be cleaned off.
After going through the necessary number of dies for the specific case being made, the case is trimmed and the head formed using a bunter. Several things happen simultaneously when forming the head. The primer pocket is formed, and the headstamp imprinted. Heading also work hardens the base of the case, but the base is left hard going forward.
After heading, the extractor groove is lathe turned and the rim thickness set. The flash hole is punched, the case is given a final trim and anneal, and if it’s a bottle-neck case like the 5.56, the shoulder and neck are formed. After final inspection, the cases are out the door.
As you can see, there are a lot of steps and processes in between. If not done right, loaded ammo will develop case neck cracks while sitting on the shelf, or case bases may be dangerously soft.
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