An expandable writing instrument has a writing end body section and a rear body section mounted to the writing end body section for relative axial and non-rotational movement between an extended position with the rear body section spaced away from the writing end body section, and a retracted position with the rear body section closer to the writing end body section. A spring in the writing end body section biases the cartridge upwardly. A helix cylinder is mounted for rotation and axial movement in the rear body section with a peg of the rear body section sliding in a track of the helix cylinder. The track is a closed loop with left and right hand helixes so that axial movement of the rear body section causes rotation of the helix cylinder continuously in one rotational direction. A shuttle receives a cartridge body and is mounted for axial movement in the helix cylinder. A shuttle cam on the shuttle is mounted for axial non-rotational movement to the writing end body section. Shuttle cam surfaces slidably engage cam surfaces of the helix cylinder and the cam surfaces include a high point that engages a flat portion of another cam surface for moving the shuttle with rotation of the helix cylinder, to project a writing tip of a cartridge out of the bottom opening in the extended position.