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.

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.