Lee machines their mold blocks from aluminum, making them rust-resistant and highly durable. Aluminum molds offer faster heat-up and cool-down times compared to iron or steel molds, allowing bullet casters to produce quality bullets in just a few casts. Additionally, aluminum blocks are one-third the weight of their iron or steel counterparts, reducing fatigue during prolonged casting sessions. To ensure the highest-quality bullets, mold block halves are CNC-machined for superior roundness and precise size control. The listed weights are based on Lee's recommended alloy, but actual weight may vary depending on the alloy mixture used.