The toy is a cap gun, made of chrome-plated die-cast metal with plastic parts. The barrel recoils with each cap fired. The pistol grip is embossed with sci-fi imagery such as planets, stars, etc.
The gun is approximately 10" in length.
The box for the gun, curiously enough, has become something of a pop-culture sensation. Its depiction of a number of (presumably) outer-space outlaws, each armed with his atomic pistol and flying above the earth, is widely available as a pop-art print from fine art retailers. The artwork is a perfect example of the sort of optimism attached to science and the space program during this time.