The Takedown Pin on an AR15 rifle is the first pin you push in when you want to separate the lower receiver from the upper. The Takedown Pin is located above the grip and is held in place by a small pin and spring, called the Takedown Detent and Detent Spring, which are hidden inside the Receiver. It’s a pretty simple 3-piece mechanism that usually works well to allow you to push the pin out far enough to release Upper without letting the pin separate from the weapon and get lost.
It is fairly easy to lose the Takedown Detent and Detent Spring. It usually pops out if you take off the stock without knowing there’s a spring loaded pin held in place. If you lose the Detent and Spring you can still fire the weapon but you risk having the Takedown Pin come loose while in the field. That would obviously be bad.
Since the Takedown Pin is spring loaded it is designed to stay attached to the lower when released. It does this by pushing the Takedown Detent into a groove that is cut into the shaft of the Takedown Pin with the Takedown Pin Spring that is embedded in the receiver.
The groove on the Takedown Pin holds the Takedown Detent in place and prevents the Takedown Pin from being separated from the receiver and getting lost.
In order to fix this you need to get a new Takedown Detent and Takedown Spring, provided you haven’t lost the Takedown Pin. These can usually be had in a set from a local gun store, gun show or on-line. The cost is minimal, probably less than $10.00 for the Detent and Detent Spring.
To install the new Takedown Pin you’ll want to remove the Buttstock. There is a Philips-head screw on the Buttstock Pad that holds the Buttstock to the receiver. Remove that screw then slide the Buttstock straight back and off the Receiver Extension tube.
Important: If you still have the Takedown Detent and Detent Spring in place be careful you don’t lose them when you loosen and remove the Buttstock!
Look on the end of the receiver that you just exposed; you’ll see a small hole on the lower right area of the face that is connected to the hole the Takedown Pin slides through. That’s where the Takedown Spring and Takedown Detent will go.
Insert the Takedown Pin into the receiver with the slot facing the back of the weapon. Slide the Detent Pin into the hole you exposed by removing the Buttstock and make sure it is seated in the slot in the Takedown Pin. Then slide the Detent Spring in after the Detent Pin. Now slide the Buttstock back on the same way it came off and carefully compress the Detent Spring by butting the Buttstock against the back of the receiver and re-install the screw in the backplate.
Check the action of the Takedown Pin by sliding it in and out making sure it does not come loose and that it freely moves to the locked and released positions. When you’re comfortable that the pin is working and will not come loose again you can re-attach the upper again and you’re done!
As you can see it’s pretty easy to fix the Takedown Pin by yourself. Besides the parts you will need a screw-driver to take the Buttstock off and everything else can be done with no tools.