An attachment for a semi-automatic firearm converts the firearm into a weapon of rapid fire. The attachment includes a crank driving a cam assembly in which the cam is mounted for limited relative movement to the crank. A spring in the attachment acts to bias the cam assembly relative to a follower to a safe non-firing position. The follower is acted on by the cam assembly to manipulate a linkage for pulling a trigger of the firearm in response to rotation of the crank. The follower and cam are designed to minimize the time of maximum movement of the follower thereby maximizing the time allowing the firearm to reset. In one embodiment, a timing mechanism is provided to insure that the bolt of the firearm is in its chambered position before the cam is allowed to actuate the firearm trigger.