Most people believed that it is the compressed air pressure that pushes the bullet up the barrel of an airgun or air gun, which may be correct to some extent, but this still does not provide an accurate answer. The truth is, airguns work and fires when the pellet gets powered as a result of the momentum from air molecules. It proves true for most spring airguns, as well as for Pre-Charged Pneumatic (PCP) air rifles as these airguns work in the same manner.
Thus, airguns are any weapon that works by propelling bullets pneumatically with compressed air, mechanically pressurized without requiring any chemical reactions, unlike firearms. The momentum in the airgun that propels the pellet is the momentum of any of the air molecules that build up the air.
In the airgun, inside the compression compartment is a spring-powered piston that connects to the barrel through an air transfer port. When the airgun is loaded, the piston withdraws into the compression chamber, and the air gets drawn through the transfer port. Tightly compressed air forces the barrel when the airgun fires.
When you load a bullet into the barrel of the airgun, it forms an airtight seal. After filling in the barrel with a pellet, the piston goes ahead and squeezes the air in front of it, thus increasing the pressure on the bullet till it overcomes the pellet’s resistance. Therefore, the pellet gets propelled by a small amount of compressed air at a very high atmospheric pressure. As a result, the bullet began to move down the barrel, accelerating as quickly as it can travel.
Ahead of taking a shot, the air that propels the pellet stays compressed in the airgun but certainly does not remain still. It serves as an active mass of molecules; as some of these atoms keep on vibrating, some turning, while some continue revolving around each other. Besides, there are some exceptional molecules, which keep moving swiftly, colliding with other molecules similar to dodgem cars.
All airguns essentially compress a large quantity of air at atmospheric pressure into a minimal amount, compressing the air molecules close together, thus allowing the mass of atoms to carry sufficient momentum to the pellet.
As a result, the molecules clash with the edge of the bullet and fire off at a somewhat lower velocity during the process, which then transfers some of their forces to the pellet. The momentum is the result of mass and high speed. As the mass of a single air molecule is weak, a collective effort of these molecules is needed to gather sufficient energy to push the pellet, or they should propel at powerful velocity.
Thus, learning the basics about what forces the pellet can be helpful when you are trying to understand how airguns work and why it functions the way it does, such as why changes work, and why things fail sometimes. Nevertheless, it does not explain each intricacy as it is particularly one of the factors that administer the shot round. This factor is what makes the working of the airgun so appealing for most people with curious minds.