How Cartridge Brass is Made
Precision shooters favor premium brass from Lapua, Norma, or RWS. (Lake City also makes quality brass in military calibers.) Premium brass delivers better accuracy, more consistent velocities, and longer life. Shooters understand the importance of good brass, but many of us have no idea how cartridge cases are actually made. Here’s how it’s done.
The process starts with a brass disk stamped from strips of metal. Then, through a series of stages, the brass is extruded or drawn into a cylindrical shape. In the extrusion process the brass is squeezed through a die under tremendous pressure. This is repeated two or three times typically. In the more traditional “draw” process, the case is progressively stretched longer, in 3 to 5 stages, using a series of high-pressure rams forcing the brass into a form die. While extrusion may be more common today, RWS, which makes some of the most uniform brass in the world, still uses the draw process: “It starts with cup drawing after the bands have been punched out. RWS cases are drawn in three ‘stages’ and after each draw they are annealed, pickled, rinsed and subjected to further quality improvement measures. This achieves specific hardening of the brass cases and increases their resistance to extraordinary stresses.” FYI, Lapua also uses a traditional draw process to manufacture most of its cartridge brass (although Lapua employs some proprietary steps that are different from RWS’ methods).
After the cases are extruded or drawn to max length, the cases are trimmed and the neck/shoulder are formed. Then the extractor groove (on rimless cases) is formed or machined, and the primer pocket is created in the base. One way to form the primer pocket is to use a hardened steel plug called a “bunter”. In the photos below you see the stages for forming a 20mm cannon case (courtesy OldAmmo.com), along with bunters used for Lake City rifle brass. This illustrates the draw process (as opposed to extrusion). The process of draw-forming rifle brass is that same as for this 20mm shell, just on a smaller scale.
River Valley Ordnance explains: “When a case is being made, it is drawn to its final draw length, with the diameter being slightly smaller than needed. At this point in its life, the head of the draw is slightly rounded, and there are no provisions for a primer. So the final drawn cases are trimmed to length, then run into the head bunter. A punch, ground to the intended contours for the inside of the case, pushes the draw into a cylindrical die and holds it in place while another punch rams into the case from the other end, mashing the bottom flat. That secondary ram holds the headstamp bunter punch.
The headstamp bunter punch has a protrusion on the end to make the primer pocket, and has raised lettering around the face to form the headstamp writing. This is, of course, all a mirror image of the finished case head. Small cases, such as 5.56×45, can be headed with a single strike. Larger cases, like 7.62×51 and 50 BMG, need to be struck once to form a dent for the primer pocket, then a second strike to finish the pocket, flatten the head, and imprint the writing. This second strike works the brass to harden it so it will support the pressure of firing.”
Forum member Ray Meketa (aka Cheechako) is a cartridge collector, and he is very knowledgeable about the various processes employed for cartridge manufacturing. Here are some of his observations:
Today, many, if not the majority of, brass cartridge cases are made by an extrusion process. Essentially, extrusion means that a solid brass slug (or a heavy brass cup) is extruded or squirted through a die under tremendous pressure which causes the brass to flow. Extrusion is used to form many everyday products from rain gutters to spaghetti and has made most other forming methods obsolete.
Most cases can be brought to their final dimensions by two or three successive extrusion operations as compared with five or six draws as shown for the 20mm case above. This results in an economy that manufacturers cannot overlook.
Extrusion of brass cartridge cases is not new. Some WW I .30-06 cases were made that way. In the 1960s a similar process called “Forged From Solid” was used by makers of custom ammunition and most of the low-pressure black powder cartridge cases of the last 20 years or so were also made using similar processes.
With today’s high tech machining options, making a headstamp bunter is a simple job that only a few years ago took considerable skills by tool and die makers. Bunters of past years were valuable tools and cartridge manufacturers were not about to throw them away unless absolutely necessary. Many times a bunter was adapted for use in headstamping other cartridges by simply grinding away part of the lettering. For example, during WW II when billions of cartridge cases were required, a “43″ (1943) bunter was reused in 1944 by simply grinding away the 3 leaving the single number 4 which indicated a 1944 case. Many other examples of altered bunter use are known.