Which statement best describes the recommended sizing of primer charges relative to the borehole?

• A. They should be larger than the borehole
• B. They should be smaller than the borehole
• C. They should be about the same size as the borehole
• D. They should be much smaller than the borehole

Answer: (C)

The goal is to ensure the primer’s energy is transmitted efficiently into the main charge and the rock. When a primer fits the borehole snugly—about the same diameter as the borehole—it seals well and couples energy into the surrounding material rather than escaping around the sides. This leads to more reliable initiation and better overall breakage.

If the primer is smaller than the borehole, there’s an annular gap that lets energy leak and can weaken initiation, producing uneven fragmentation or misfires. A primer larger than the borehole isn’t practical to place and can cause damage or improper initiation. So, sizing primer charges to roughly match the borehole diameter provides the most consistent, efficient performance.