Abstract: A replacement takedown pin for a firearm of the type having an upper receiver, a lower receiver, and a rear takedown pin mechanism comprises an elongated cylindrical body having a pin capture mechanism at a distal end thereof, a knob at a proximal end thereof, and a clearance notch at a central portion thereof. When the knob is in an engaged rotational position, the notch of the cylindrical body is misaligned with the catch of the takedown pin mechanism, preventing the upper receiver from being removed from the lower receiver. When the knob is rotated to a disengaged rotational position, the notch of the cylindrical body becomes aligned with the catch of the takedown pin mechanism of the firearm, thereby allowing the catch to pass the takedown pin and allowing the upper receiver to be separated from the lower receiver.