A simple yet elegantly elaborate solution to a classic problem; M4 receiver front pins can become loose over time but this one can be tightened to lock in place.

will also tighten a loose body and reduce wobble

The shaft of the pin is three cylindrical segments separated by angled edges all wrapping around a central screw. The pin slides into the receiver as normal but when the screw (hex / allen) is tightened the central segment is forced out of alignment sideways which locks the pin inside the receiver (until the screw is loosened).

This magic little pin is offered by G&P for both gas and AEGs with different versions for different types of models.