Abstract: A modular adapter for a firearm receiver that has a threaded socket, such as an AR-platform rifle, has a receiver surface with a threaded plug that is rotatable and configured to engage the threaded socket of the firearm receiver, a distal surface that provides a means to attach an accessory, e.g., a portion of MIL-STD 1913 accessory mounting rail, or function as an end cap, and a knob-driven mechanism, i.e., a mechanism that a hand could operate to cause the threaded plug to rotate and thereby engage the receiver, positioned between the receiver surface and the distal surface, and connected to and able to rotate the threaded plug.