An air shot or priming mechanism for an injection pen. The injection pen comprises a cartridge-holder and a housing. The cartridge-holder interacts with a nut-member capable of driving the piston rod forward when relatively rotated. The cartridge-holder is rotational coupled to the housing such that rotation of the cartridge-holder relatively to the housing causes the nut-member and the piston rod to rotate relatively, thus the relative angular rotation between the cartridge-holder and the housing determines the size of the air shot so performed.