An airsoft gun relies on a spring to propel a projectile from the barrel; no gunpowder is used. To reduce the FPS (feet per second) the projectile travels, users can modify the gun. There are a few methods to accomplish this, with some requiring a partial disassembly.
Disassemble the gun with a Phillips screwdriver so that you can remove the piston chamber (sometimes called a "firing chamber") from the top of the gun. Clamp the jaws of pliers around one end of the spring inside the chamber. Pull the end of the spring off the rod that is against the inside of the chamber. Repeat this with the other end of the spring. Pull the spring out of the chamber. Replace the spring with one that has the same dimensions but is rated by the manufacturer with a lower "compression" rate. This will ensure that the projectile is fired at a slower FPS. Place the chamber back into the top of the gun and reassemble the parts together.
Partially unscrew the "bleed" valve on the tank providing a gas-firing gun with power. The gas now entering the firing mechanism of the gun will be less than optimally efficient, resulting in a less-powerful activation of the firing mechanism.
Replace the pellets, or BBs, with heavier-weighted ammo. These pellets will not travel with the same FPS as those rated for the gun being used, and so will travel less distance.
Always remove the magazine clip prior to making any modification, no matter how minuscule, to an airsoft gun.