Now that we’ve finished installing our safety selector and pistol grip, we’ve made it to the final steps in our AR-15 lower receiver build: installing the receiver extension, the rear takedown pin, and, finally, the buttstock. To look over what tools and parts I’m using for this build, check out this post. If you’d like a visual reference, the time stamps at each step heading correspond to this video.
(16:50) Step 9 – Receiver Extension and Rear Takedown Pin
These instructions will be for a lower receiver build that will be utilizing a collapsible stock
Now, screw the receiver extension clockwise into the lower receiver until it just touches the retainer. Grab that trusty pencil again and use it to depress the retainer so you can finish turning the receiver extension. Continue to turn the receiver extension clockwise until it’s trapping the buffer retainer and is in the correct position, with the flat side facing downward and level.
Place the takedown pin into the lower receiver with the “channel” facing the rear, and the large end on the right side of the lower. Next, put the takedown pin detent into the hole in the rear of the lower receiver, followed by the takedown pin spring. There should be a portion of the spring sticking out.
Slide the lower receiver end plate over the lower receiver extension first, followed by the castle nut. On the castle nut, there are large notches, and small notches. You will want to place it onto the extension with the small notches facing the lower receiver. While keeping the receiver extension level, push the end plate against the spring without bending it and then tighten the castle nut by hand, spinning it clockwise.
Once you have the castle nut hand tightened and the end plate is securely held in place, use the castle nut wrench to tighten it down nice and secure. Don’t go Hulk mode on it, but it should be tight enough that you cannot loosen it with your hands.
With the hammer cocked back, and the safety selector in the safe position, slide the buffer and buffer spring over the buffer retainer and into the lower receiver extension. Slowly let it go to make sure that the buffer retainer does its job of keeping the buffer inside the lower receiver extension.
Make sure the takedown pin functions properly (by pulling it out and pushing it back in) and you are ready for the final step.
(21:21) Step 10 – Buttstock
That’s it. Congratulations, you have completed building your AR-15 lower receiver. Now take a picture of your build and share it with me and tell me about your experience while we get ready to start the AR-15 upper receiver.
