Why should I never dry-fire a spring piston airgun?

Without a pellet to push against, the piston slams forward unimpeded and the seal smashes into the end of the compression chamber at full speed.

Repeated dry-firing can shatter the piston seal, break the spring, and crack the receiver in extreme cases. A few accidental dry-fires will not destroy the gun, but make it a habit not to dry-fire. Gas piston guns are slightly more tolerant but the same rule applies.