Strike Industries’ solution to the problem of rotating and walking out AR-15 pins is pretty unique and simple as in the case of many other of their products. The pins of their anti-walk/anti-rotation system extend a bit over the lower receiver exposing a groove. The retention itself is accomplished by an external clip.
Judging by the appearance of the clip, it probably can spring open a little bit to snap on the installed pins. The anti-walk mechanism is obvious – the clip rests in the exposed pin grooves and restricts their horizontal movement. The anti-rotational function is accomplished by the geometry of those additional grooves. If you look closely at the above image you will notice that these grooves are not cut around the circumference of the pins. They are rather parallel side cuts. The result is a couple of flat surfaces, which house the clip and prevent the rotation of the pins around their axis.
Another advantage of this pin system that SI points out, is the tool-less installation and absence of screws seen in competitors’ products. However, the slots in the pin heads and the cutout in the clip are probably there to allow using a tool (e.g. screw driver) to align the pins and remove the clip respectively.
The SI pins are made of S45C steel with QPQ (Quench Polish Quench) Nitride finish. The surfaces of the pins are also polished to reduce the friction between the pins and the hammer or trigger. The pins’ diameter is .154″.
The Strike Industries anti-walk/anti-rotation pin set is now available through their website at $14.95.