The internal structure of the bb gun has several different designs. The main difference is that the compressed air source is different: in the pneumatic design, you need to manually pump the air to accumulate compressed gas. The movable lever at the bottom of the gun is able to move the small piston in the trachea. There is a check valve in the trachea that allows the airflow to flow in but not back. In this way, each time the gun pumps air, the air in the accumulator is increased, and the volume of the accumulator is fixed. Since the mass increases with constant volume, the density and pressure of the air will increase with each pumping pressure.

Another common system is the spring compressed gas design. In this type of firearm, the "pump" lever pushes a small piston backwards, while the latter compresses the spring behind it. When the piston slides back, it will be locked by a small spring-loaded pin that is wound around a small rod, commonly referred to as a spring, which holds the piston in compression by holding the piston. When the trigger is pulled, the spring is pushed out of position.

When the piston is released, the spring will stretch and push the piston forward. This will quickly compress the air inside the barrel of the BB to obtain the pressure required to push it along the barrel. Some air guns do not perform any type of pumping motion.