This slave pin is used to assemble the trigger group outside of the AR so you could the trigger, disconnector and spring as a group. Once inserted in the lower, you push the normal trigger pin in and as it goes in, it pushes the slave pin out the other side of the receiver. No more fumbling with keeping everything together during assembly! Seriously, it makes installing an AR trigger soooo much easier.
This pin is for triggers that use a normal or “small “0.154 inch diameter pin. In case you don’t know why I say that, Colt built some rifles that used a larger pin from 1991 to the fall of 2009 and only they did that to the best of my knowledge. The majority of ARs use the 0.154″ pin today — I think they all do and you can measure your pin if you want to confirm the size – it should be right around 0.154”. The large pins measure 0.169″ and these slave pins will not work in them. Geissele has a great write up on pin sizes – click here for it.