Rust prevention device for gun

A rust prevention device for guns which includes a mounting case with a mounting space and a bullet-shaped rust prevention plug mounted in the mounting space is revealed. An outer diameter of the rust prevention plug is sized to fit in a bore or a magazine of the gun. A main body of the rust prevention plug which is a molded block made of porous adsorbents or volatile corrosion inhibitors (VCIs) is exposed outside the mounting case. Thereby the rust prevention device is loaded into the bore or magazine like a cartridge and the main body of the rust prevention plug is exposed to increase surface area in contact with air in the gun. The rust prevention plug has more contact with metal parts in the gun due to diffusion in air. Therefore, the gun is better protected against rust.