What do shell casings contain that is essential for firing a bullet?

The correct answer emphasizes the essential components of a shell casing that enable the firing of a bullet. A shell casing is designed to hold three critical elements: gunpowder, a primer, and a projectile.

Gunpowder serves as the propellant that generates gas and pressure when ignited, pushing the bullet or projectile down the barrel of the firearm. The primer is a small explosive charge located at the base of the cartridge that, when struck by the firing pin, ignites the gunpowder. Lastly, the projectile is the actual bullet that is expelled from the firearm.

In contrast to the other options, the omission of one or more of these elements from the shell casing would fundamentally impair the ability of the cartridge to function correctly. For instance, a shell casing containing only a projectile or a firing pin mechanism would lack the necessary components to achieve projectile propulsion. Similarly, a casing containing a liquid propellant would not conform to the traditional design of cartridges that utilize solid gunpowder. Thus, the combination of gunpowder, primer, and projectile is what makes a shell casing functional and effective for firing a bullet.