Abstract: A reactive force compensator is provided for a projectile firing device, such as a bow or gun, on which a force is exerted in a given direction when the projectile is fired. The compensator comprises an elongate casing adapted to permit the casing to be mounted on the projectile firing device with the length of the casing parallel to the direction of said force. A weighted piston member is disposed in the casing and adapted to reciprocate therein while remaining parallel thereto. The piston preferably has an aerodynamic end, disposed rearmost with respect to said force, and a more resistive end disposed forwardmost with respect to the force. Preferably, the latter end is defined by a hollow opening endwise through the piston member.