How Are Bullet Molds Built?

Lathing The Graphite Rod

First, a rod of graphite is cut down to about 3 inches and loaded onto a lathe. Just like a pottery wheel, a lathe rotates material around its central axis while a stationary blade or grinder wears away at the piece's cross-section. Because of this constant rotation, the cross-sectional shape of the grinder can form a perfectly uniform ridge or "collar" around the piece.

The lathes used by munitions companies are completely automated, controlled by state-of-the-art engineering software. So, based on the bullet's digital blueprints, the lathe loads the graphite piece and applies a series of high-speed circular grinders that edge themselves into the spinning lathe at hyper-precise coordinates.

Once the lathe has finished, the rod features a perfect scale model of the final bullet on its tip, also known as the "bit."

Forming The Mold: Single Cavity

There are two types of molds: single cavity (used with a hydraulic press, preferred by manufacturers) and multicavity (involves pouring molten metal, used often by hobbyists). However, the main difference between the mold-making process primarily is in the number of bits and alloys used.

Known officially as "bullet swaging dies," single-cavity molds are made in one of two ways: casting and machining. For casting, the graphite bit is secured bullet-side down above an open trough. Usually 4-by-4-by-3 inches, this trough is made of ceramic materials like clay graphite and silicon carbide, which are rated for temperatures between 2,000 and 2,700 degrees Fahrenheit. Note: The melting point of graphite is almost 1,000 degrees Fahrenheit, higher than mold-grade steel. Therefore, when molten steel gets poured around the bit, the graphite will keep its shape.

Once everything is in place, special steel (e.g. A2-, D2-, S2- or S7-grade) is melted and poured into the trough until its surface reaches the upper edge of the bit. After the steel cools and hardens, its slight contraction allows the mold to be removed from the trough. The extruding part of the graphite rod is severed and a high-precision machine cuts the mold down the middle, forming two identical halves.

Forming The Mold: Multiple Cavities

To cast bullets from a molten lead-antimony alloy, a multicavity mold is the more practical tool. The setup is very similar to single-cavity casting (i.e. troughs, graphite bits, bisecting the cast steel block to create two mold halves) except that 4 to 10 graphite bits are needed. Also, because resistance to mechanical stress isn't an issue, you can choose steel, copper or aluminum alloys for the casting metals.

The graphite bits are arranged in tandem along the center of the 2-by-2-by-9-inch trough, and the molten metal is poured in. Once cooled, the block is cut in half.

References

• Bullet Swaging
• The Basics Of Bullet Mold Making
• Grades Of Mold Steel