How does a spring piston airgun work?

When you cock the gun by breaking the barrel down (or pulling an underlever or sidelever), a heavy steel spring is compressed behind a piston.

Pulling the trigger releases the spring, which slams the piston forward and compresses a column of air ahead of it. That compressed air rushes through a transfer port and pushes the pellet down the barrel. Because the piston moves before the pellet leaves, the gun has a unique double recoil pulse that demands proper technique.