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How to Install a Finished 80% Lower

How to Install a Finished 80% Lower

Congratulations, you’ve completed your 80% lower by machining it and installing your Lower Parts Kit! Now all you need to do is install your buffer tube, buffer spring, buttstock, and upper receiver. Rest assured, installing your LPK was the most difficult part! If you haven’t gotten this far, be sure to pick up the appropriate 80 lower and jig (we have combo kits!). Here are the last steps to pair your 80% lower with your buffer system and upper receiver.

AR-15 buffer parts needed

First, you’ll need to install your completed 80% lower’s buffer system. The buffer system controls the recoil action of your bolt carrier group, allowing rounds to be ejected and chambered. The weight of the buffer will determine how much recoil energy is absorbed or transferred back into the bolt carrier group.

Rifle-length gas systems favor lighter buffers. Mid-, carbine-, and pistol-length gas systems favor heavier buffers to compensate for the added recoil energy provided by those shorter gas systems. An AR-15’s buffer system comprises a few components. If you need a buffer system, we’ve got you covered with our standard AR-15 Lower Parts, Butt Stock and Buffer Tube Assembly.

Adjustable Carbine Stock & Buffer Tube Assembly - Blog Photo 1

  • Buttstock
  • Buffer
  • Buffer spring
  • Buffer tube
  • Castle nut
  • Receiver end plate

80% lower kit parts needed

You’ll also need the following components provided in your Lower Parts Kit. If you don’t have one yet, grab our Premium LPK here!

AR-15 Premium Lower Parts Kit - Blog Photo 2

  • Buffer Retainer
  • Retainer spring
  • Rear takedown pin
  • Takedown pin detent
  • Takedown pin detent spring

Installing your 80% lower’s buffer system

  1. Thread the castle nut onto the buffer tube. The crown of the nut should face the receiver.
  2. Slide the end plate onto the buffer tube so it is resting on the castle nut. The raised side of the plate should face the receiver. The receiver has an indentation for the end plate to be seated.
  3. Thread the buffer tube with nut and plate into the lower receiver. Stop threading once the tube is resting against (but not on top of) the buffer retaining hole on the receiver’s side of the buffer tube housing.
  4. Insert the buffer retainer and spring into the hole in the rear of the receiver, underneath the end plate, depressing it into the hole fully.
  5. Continue threading the buffer tube until the retainer is trapped by the end of the tube.
  6. Next, insert the rear takedown pin into the rear-facing hole on the right side of the receiver, between the end plate and the receiver.
  7. Last, insert the rear takedown pin spring. The pin and spring will protrude from the hole until fully compressed by the end plate being seated onto the receiver with the castle nut tightened.
  8. Press the end plate into the lower receiver so it is pressing the rear takedown pin and spring into the receiver fully.
  9. While holding the end plate against the receiver and compressing the rear takedown pin and spring, screw the castle nut against the end plate*, locking it and the rear takedown pin and spring in place.

*We strongly recommend using an Armorer’s Wrench and blue Loctite to tighten the castle nut fully.


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