What component is responsible for propelling a bullet out of the barrel?

The correct answer is propellant/expanding gases because this component plays a fundamental role in the operation of a firearm. When the primer is struck by the hammer, it ignites the gunpowder, which is the propellant. This ignition creates rapidly expanding gases that exert pressure on the base of the bullet, forcing it down the barrel and out of the firearm at high speed.

The expanding gases are critical as they provide the necessary force to overcome inertia and the resistance of the bullet in the barrel, propelling it towards the target. This concept is essential in understanding how firearms operate and is a key principle in ballistics.

Other components mentioned, such as gravity or friction, do play roles in the overall mechanics of shooting, but they do not directly propel the bullet out of the barrel. Gravity acts on the bullet after it exits the barrel, and friction can slow the bullet down within the barrel but does not contribute to its propulsion. The mechanical action of the hammer is important for igniting the primer but is not responsible for the propulsion of the bullet itself. Therefore, the propellant/expanding gases are the driving force behind the bullet's launch.