What causes a bullet or projectile to travel down the barrel of a firearm and exit at the muzzle?

The correct answer highlights the critical role of expanding gases from the burning gunpowder in propelling a bullet down the barrel of a firearm. When the trigger is pulled, the primer ignites the gunpowder, creating a rapid combustion reaction. This reaction produces a significant amount of gas that expands rapidly.

As the gases expand, they create high pressure within the chamber of the firearm, which pushes the bullet forward through the barrel. The design of the firearm's barrel, which typically has a narrower diameter, helps to channel that pressure effectively and increases the bullet's speed as it travels toward the muzzle. This principle is fundamental to the operation of firearms and is what allows a bullet to exit with such velocity and accuracy.

Understanding this mechanism is essential for safe firearm handling and operation, as it illustrates the importance of the ammunition and the firearm's design in the shooting process.