Various mechanisms are provided for use in a surgical instrument. A shaft and an engaged lost motion operating knob are provided for rotation relative to a handle portion. A latch system prevents rotation of the shaft unless the shaft is rotated by the knob. A cartridge is provided for rotation at the distal end of the shaft. Engaging structures are provided for preventing rotation of the cartridge relative to the shaft when axial forces are applied to the cartridge. A loading tool is provided for engaging the cartridge to install the cartridge on the instrument. The cartridge includes a washer for engaging a carrier on which the cartridge is mounted so as to prevent rotation of the washer. The washer is adapted to engage a fastener advancing member in a first angular orientation and is disengaged when the cartridge is rotated to establish a second relative angular orientation. In the first angular orientation, the fastener advancing member is restrained against longitudinal movement to assist in installation of the cartridge onto the instrument. The distal end of the instrument can be articulated, and a helical gear operator is provided for effecting the articulation. The helical gear pitch decreases in a direction such that the mechanical advantage is increased as the articulation angle increases. A trigger actuating system includes a trigger lever with a cam channel, a pin in the cam channel, and a link connected at one end to the pin and at the other end to a reciprocatable driver. A frame is provided with a guide channel for guiding an end of the pin. The trigger lever is pivotally mounted to the frame and biased to a normally unactuated position.