How to Build an AR-15 Lower Assembly

A step-by-step guide to building an AR-15 lower.

Lower Build Kits

Lower Assembly Parts List


Lower Parts Kit Includes:
    • Pistol Grip
    • Screw Pistol
    • Grip Lock Washer
    • Trigger Guard
    • Trigger Guard Roll Pin
    • Standard Trigger
    • Trigger Spring
    • Trigger Pin
    • Hammer

    • Hammer Spring
    • Hammer Pin
    • Disconnector
    • Disconnector Spring
    • Safety Selector
    • Selector Spring
    • Selector Detent
    • Bolt Catch
    • Spring Bolt
    • Catch Bolt
    • Catch Roll Pin
    • Magazine Catch Button
    • Magazine Catch
    • Magazine Catch Spring
    • Front Pivot Pin
    • Pivot Pin Detent Spring
    • Takedown Pin
    • Takedown Detent
    • Buffer Retainer Pin
    • Buffer Retainer Spring

Recommended Tools


Getting Started

Install Front Detent Spring, Detent, and Front Pivot Pin

  • Insert the front small detent spring into the channel on the front of your lower receiver.
  • Press the small detent into place
  • We recommend using a pivot pin tool to hold the detent and spring into place to make things easier. If you’re not careful, the detent and spring can fly off into the abyss.
  • Insert the pivot pin into place. The detent should sit inside the inner channel of the pivot pin.

Install Magazine Release

  • Using your magazine release spring, button, and magazine release bar. If you’re using a right-handed lower receiver, place the magazine release bar from the right side, then slide the spring over the bar from the left side
  • Start the magazine release button on the threads of the magazine release bar. Once it is started and you’re not able to turn it anymore you can press the magazine button on the receiver, and continue turning the magazine release bar from the other side.
NOTE- the mag release bar goes in on the left side and the button goes on the right side of the lower.

Install Bolt Release

  • The bolt release mechanism uses a roll pin, Bolt Release Spring, Bolt Release Detent, and the Bolt Release.
  • Using a roll pin punch, get the roll pin started into the first tab.

  • Insert the spring and detent into the channel
  • Insert the bolt release and hold it in place with one hand to depress the spring. Push the bolt in and line up the holes on the bolt release with the holes on the receiver.
  • Once the holes are lined up, Using a punch or a pair of non-marring pliers, press the roll pin all the way across.
  • Note: Some lower receivers have a threaded bolt release. Having a threaded bolt catch replaces the need for a roll pin.

Install Trigger Guard (if applicable)

  • There are a few different styles of trigger guards, some lower receivers have the trigger guard already machined into it.
  • Other aftermarket trigger guards just snap into place
  • The most common trigger guard style that comes with a standard Lower parts kit will be held in place using a roll pin.
  • This roll pin can be installed by placing the lower on its side and gently pressing the roll pin in place.
  • Make sure that everything is lined up properly to avoid bending or breaking the tabs on the lower receiver.

Install Trigger Assembly

Install Trigger and Trigger Spring

  • Take your trigger spring and slide it over the arms on either side of the trigger. The two legs of the trigger spring will be pointed downwards so they’ll sit flat in the receiver, the and top loop portion of the trigger spring sits under the front belly of the trigger as shown in the photo

Insert Disconnector Spring

  • The channel on the top of the trigger has a small hole for your disconnector spring to sit. The disconnector spring is cone-shaped. Place the wider end of the spring into the spot on your trigger, and the more narrow end pointing upwards.

Install Disconnector and Bottom Trigger Pin

  • Your Disconnector will sit inside the channel on top of your trigger, a small cut-out on your disconnector will line up for the top part of the disconnector spring to sit.
  • Insert the trigger and disconnector assembly into the lower receiver, and press down on the unit until the forward bottom hole in the lower receiver lines up with the trigger and the disconnector holes
  • Insert the Trigger pin into the side making sure everything lines up. There may be some difficulty with getting everything to line up. Don’t use too much force to insert the pin, but a small mallet can be used to help guide the trigger pin in so it sits flush on both sides of the lower receiver
  • After getting the trigger and disconnector into place, the trigger should pivot freely.

Install Hammer Spring (Proper Orientation)

    • To install the hammer, place the legs of the spring downwards behind the trigger pivot. Load the spring while pressing downwards until the hole on the hammer assembly lines up with the hammer pin hole

Install Safety Selector

  • Slide the safety selector switch into place so it sits flat against the side of the receiver.


Install Grip and Safety Selector Detent

  • The safety selector is held in place using the detent and detent spring held in place by the grip.
  • Insert the detent into the small hole in the bottom of the lower receiver, slide the detent spring into the hole in the top of the grip and slide the grip into place
  • Using a long Allen key, the grip screw, and the lock washer, tighten the grip onto the lower receiver. Make sure that the detent spring does not get pinched or bent in between the lower receiver and the grip
  • After tightening the grip, you can test the function of the safety selector switch.

Install Buffer Assembly


Insert Buffer Retainer Spring and Pin

  • Place the buffer retainer spring and pin it into the hole towards the rear of your receiver where you’ll be threading on your buffer tube

Install Rear Takedown Pin, Spring, and Detent

  • Your rear takedown pin spring and detent are held in place using the end plate of your rifle. Before you turn your buffer tube into place, you’ll need to insert the detent, spring, and takedown pin. Then use a small punch to depress the spring into the receiver enough to be able to twist the buffer tube and end plate into place over the top of the spring. Be very careful not to pinch, fold, or bend that spring as you attempt to cover it with the end plate

Thread in Buffer Tube, End Plate, and Castle Nut

  • Twist your castle nut onto your buffer tube, and twist it on all the way back so you have room to slide your end plate on.
  • Twist your buffer tube onto the threads of your rear receiver until the notch on your buffer tube engages with your buffer retainer pin. Depress the retainer pin and spring while twisting the buffer tube into place. The buffer retainer pin will snap into place inside the end of the notch on your buffer tube.

Tighten Castle Nut

  • Using an armorers wrench, tighten the castle nut until it is snug. (Although proper torque specs is a very popular debate topic, most AR-15 Mil-spec manuals recommend torquing your castle nut between 38 to 42 ft/lbs)
  • It is recommended to stake the castle nut or use your preferred thread locker to prevent the castle nut from coming loose.

Buffer Spring and Buffer Install

  • Insert the buffer spring into the buffer tube, then slide the narrow end of your buffer inside the other end of the spring.
  • Press the hole unit into the buffer tube until the buffer snaps in place behind the buffer retainer pin.
  • If you need to remove the buffer, press down on the buffer retainer pin and everything will spring out.

And you're all set! You should now have a fully assembled lower receiver ready to pair with your Complete Upper Build.

Any additional tips you would like to share with your fellow gun builders? Leave a comment below!

Did one of your springs or detents launch off into oblivion never to be found again? Don't worry, it happens! We've put together a "Lost Parts" kit just for you. That can be found HERE.

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