Why is gunpowder so explosive?

Gunpowder of itself is not actually explosive, but rather flammable. Gunpowder is designed to burn at a very specific, controlled rate in order to lend a certain consistency to the various products that use gunpowder (that is, if you were to build fireworks or explosives like dynamite one could depend upon the same burn rate for a particular kind of gunpowder). If gunpowder were to be poured on the ground and lit, it would simply burn, sometimes rapidly, but no explosion would occur. The 'explosive' nature of gunpowder comes from the release of a variety of gases produced during the burning process. This gunpowder-burning process is initiated by igniting what is known as a primer: a veryrapidly flammable (explosive, even!) substance that reacts to various energy input such as heat or shock - a fuse or firing pin). Typically, these gases are contained or restrained (e.g., in a shotgun shell or a firecracker) until a predictable amount of gas has accumulated, sufficient to cause a sudden, almost instantaneous release of this gas through a predetermined outlet: the forcing out of the paper or plastic wadding of a shotgun shell, or simply the restraining force of a firecracker's paper shell. The explosion (near-instantaneous release) of these gases is what propels shotgun pellets or fireworks particles outward in the familiar explosive pattern.

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