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Why did black-powder makers produce grains of uniform size for firearms?

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coal miner loading a drill hole with an explosive — explosive A coal miner loading a drill hole with a water gel explosive called Tovex.

Black-powder makers produced grains of uniform size to control the burning rate of the explosive, which is a surface-dependent phenomenon.

In its original powdered form, black powder burned too rapidly to be a safe propellant for firearms, as the high speed of combustion created excessive pressure within the gun barrel. Because fine granulation increases the surface area and results in a faster burn, manufacturers began producing larger, uniform grains in the 15th and 16th centuries to slow the combustion process and improve ballistic safety.12

By varying the size of the granules, makers could precisely regulate how quickly the powder burned. This control allowed for more consistent performance and, in later developments, enabled the creation of grains shaped to provide a progressively greater burning surface, ensuring maximum energy release only after the projectile had begun moving down the bore of the gun.1