Lee machines its mold blocks from aluminum, offering superior rust resistance compared to iron or steel. Aluminum molds heat up and cool down more quickly, allowing bullet casters to produce quality bullets after only a few casts. Additionally, aluminum blocks are one-third the weight of their iron or steel counterparts, reducing fatigue during long casting sessions. To ensure optimal bullet performance, the mold block halves are CNC-machined for exceptional roundness and precise size control. The listed weights are based on Lee's recommended alloy, though actual weight may vary depending on the specific alloy mixture used.