A firearm uses pressure from what to force a projectile through and out of its barrel?

In a firearm, the mechanism that propels a projectile out of the barrel relies on the rapid expansion of gases generated from a chemical reaction. This reaction is caused by the ignition of gunpowder (also known as smokeless powder), which is the propellant in most modern ammunition. When the gunpowder ignites, it produces a large volume of gas very quickly. This sudden increase in gas pressure pushes the bullet forward through the barrel and out into the target.

The other options do not accurately describe the process involved in firing a firearm. Fuel, while potentially related to combustion in other contexts, is not directly applicable to the mechanism of projectile propulsion in firearms. Air does not provide the necessary pressure to propel a bullet on its own without the combustion process. Lastly, water is not used in the firing process of conventional firearms; it does not create the explosive reaction required to generate the pressure needed for launching a projectile.