Anti-Rotation Pins are normally not a necessary modification to an AR-15 lower, but their are some exceptions. What the pins do is prevent the hammer and trigger pins in the AR-15 (or M-16) from rotating. A steel pin in an aluminum receiver can rotate and elongate the holes. This causes malfunctions and the destruction of a lower.
Normally the design of the pins prevents this, and a new lower doesn’t cost much more than the pins. However, many people that own registered pre-1986 lowers for fully automatic AR pattern rifles add them because their lowers are irreplaceable. If their lowers pins elongate they lost a machine gun.
I add one on my polymer lower because the hammer pins on mine wallowed out after less than a magazine worth of rounds.
One thing about installing the Elfman Tactical Anti-Rotation Pins in a freedom15 polymer AR-15 Lower, there is a strengthening rib molded on the lower that you will need to chisel off in order to install the anti-rotation pin. It was not hard to do, but took a sharp chisel and permanently altered the lower.
I used to link directly to the item on Amazon so you could purchase it easily, but they have, in their infinite liberalism decided to remove firearm parts as they find them – they did this with tannerite also.