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How does black powder act as a propellant in a gun?

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Black powder acts as a propellant in a gun by undergoing rapid combustion when ignited by flame or heat. Composed of a mixture of saltpetre, charcoal, and sulfur, the substance burns to produce a significant volume of gaseous products alongside solid residue.1

When this reaction occurs within the confined space of a gun's breech, the rapidly expanding gases create high pressure. This pent-up force is directed against the base of a bullet or artillery shell, forcefully ejecting the projectile down the barrel.1

Because the burning of black powder is a surface phenomenon, its performance is highly dependent on granulation. Fine powder burns faster than coarse powder, which can create excessive pressure; therefore, manufacturers historically produced large, uniform grains to control the burn rate and ensure the propellant functioned safely and effectively as a propellant. While largely replaced by smokeless powder in modern firearms, black powder remains essential for specific applications like ignition charges and fuses.1