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Installing An AR-15 Lower Parts Kit

Nothing compares to the sense of accomplishment we gun enthusiasts feel when we build, modify or repair our own firearms. Although the idea of tinkering with the inner workings of a gun may seem daunting, you may be surprised at how easily you can persevere with a little guidance – and a lot of research!

Today, we would like to help you tackle the task of installing a lower receiver for an AR-15. Like most firearm projects, installing your own AR-15 lower parts kit is essentially a “puzzle with a payoff,” provided you have a bit of determination. Kits are available through a host of vendors, and will cost you around $70.

Before You Get Started

Nothing is more frustrating than not having the proper tools when you need them. Although you may not utilize everything we recommend, we are providing you with an extensive list in order to head off unforeseen problems you may encounter.

Essential Tools:

  • small punch
  • hammer

Highly Recommended Tools:

  • variety drill bits
  • pin vice
  • trigger guard installation fixture
  • holding fixture
  • hammer block
  • variety punches
  • pivot pin installation tool
  • AR-15 combination wrench
  • marker for touch-ups
  • masking tape
  • gun oil
  • grease
  • tapping oil
  • Loctite
  • screwdriver set

Prepare Lower Receiver and Parts Kit

Examine the lower receiver to locate any evidence of damage or imperfections. Check the pin holes to ensure they are free of debris and machining burrs. For extra assurance, you can use a proper-fitting drill bit and pin vice to clear each hole in the receiver prior to any parts installation. A drop or two of tapping oil can be used to clean the threads of the pistol grip screw hole.

Arrange your kit parts on a flat surface where you can easily identify and retrieve them as needed. A great reference photo to assist you in organizing your kit can be found at https://www.ar15.com/content/guides/assembly/lower/lowerKit.gif

Wipe down the contents of your kit to remove excess oil, if necessary. In the event your parts arrive to you without any oil pre-applied, however, you may find it beneficial to place a drip of gun oil on each pin to aid installation.

With your preparation complete and your tools poised, you are now ready to begin your project!

Trigger Guard Assembly

On the front of the trigger guard, you will find a pre-installed, spring-loaded detent. Place the detent into the front of the receiver, between the two flanges or “ears.” Depress the detent with a punch to slide it into place. 

Before attempting to install the rear of the trigger guard, place the receiver onto a trigger guard installation fixture. This helps to protect the flanges from breaking, which would render your entire lower receiver useless. 

Once secured in the fixture, place the rear of the trigger guard between the flanges. Using a roll pin starter punch, gently tap a roll pin into the aligned holes with a hammer. Once the roll pin is started, use a standard roll pin punch (which has a protruding end that fits into the roll pin) to finish tapping the roll pin into place. Both ends of the pin should be flush with the flanges when the pin is fully seated.

Magazine Catch Assembly

Begin by placing masking tape around the raised areas of the receiver where you will be working. This will help to protect the surface while you install the magazine catch, spring and button.

Insert the magazine catch into the left side of the receiver. Lubricate the magazine catch spring. On the right side of the receiver, place the magazine catch spring over the protruding end of the magazine catch. The magazine catch button will then need to be screwed onto the same end of the magazine catch that you placed the spring onto a moment before. Turn the button clockwise to seat.

Align the button with its hole in the receiver and utilize a punch to depress the button far into the receiver. On the left side of the receiver, while still depressing the magazine catch button, rotate the magazine catch clockwise until the innermost portion of the button is flush with the side of the receiver.

Test the magazine catch assembly by making sure an empty magazine will hold in place as intended and that it releases with ease. It is also wise to ensure the release button functions without any dragging or obstruction.

Remove your protective masking tape, and your magazine catch assembly is complete!

Bolt Catch Assembly

Installing the bolt catch will require four parts: a release/catch, roll pin, spring and plunger. Put a drop of oil on the roll pin before you begin, because it will fit tightly and at an awkward angle. Applying a slight bit of grease to the plunger and spring will also aid your effort. Lastly, protect your lower receiver with masking tape in order to avoid damaging the surface during this assembly.

Begin by placing the spring into its hole on the left side of the receiver. The thinner portion of the plunger goes into the same hole as the spring, with the thicker end of the plunger facing outward. Next, place the bolt catch into the hole, over the plunger.

Affix the end of the roll pin to a roll pin punch using a small piece of tape. The leading end is beveled. Start the roll pin into the rear of the receiver just slightly. Insert a second punch into the front of the receiver in order to align it with the roll pin. You will then be able to tap the roll pin through the receiver and the bolt catch from the rear. When it is properly seated, the roll pin will slightly protrude on both sides.

Ensure your bolt catch moves smoothly. We also recommend you insert a magazine to check that the bolt stop engages and moves the bolt stop upwards.

Pivot Pin Installation

Begin by placing a small amount of oil on the pivot pin and lightly greasing the spring and detent. Insert your pivot pin installation tool into the pivot pin holes on the front of the lower receiver, making sure to align the hole in the tool with the detent hole. You will then be able to insert the spring into the top of the tool hole, followed by the detent.

Using a punch, press the detent through the hole of the installation tool and into the hole in the receiver. With the punch still inside the tool hole, gently rotate the punch 45 degrees in order to block the spring and detent inside the receiver. You will then be safe to remove the punch.

With the groove of the pivot pin facing upwards, slide the pivot pin into the pivot pin holes, pushing the installation tool out as the pin goes into place. Turn the pivot pin until the detent springs into the pivot pin groove. The pivot pin will no longer rotate once the detent is seated, but you will be able to slide the pin back and forth in the pivot pin holes in the front of the receiver.

Fire Control System: Hammer and Trigger Assemblies

With the legs of the trigger spring facing forward, place the trigger spring under the trigger and begin affixing the trigger spring in an upward direction. The crossbar section at the front of the trigger spring will ultimately rest just below the front of the trigger. The spiraled sections of the spring fit over the protrusions on both sides of the trigger.

Locate the disconnector spring hole on the rear, top portion of the trigger. Insert the larger end of the disconnector spring into this hole, with the spring ultimately pointing upward and perpendicular to the top of the trigger. Place the disconnector into the groove at the top of the trigger, making sure the hook of the disconnector is facing upward and forward.

You are now ready to drop your trigger assembly into your lower receiver and align it with the receiver holes. Next, press down on the disconnector in order to allow you to place the trigger pin through the side of the receiver and into the trigger and disconnector, ultimately emerging on the opposite side of the receiver.

Check your trigger functioning for smoothness and ensure the disconnector pivots as intended.

Next, take you will affix your hammer spring by placing the spirals of the spring over the protrusions on both sides of the hammer. The crossbar section of the spring will face the rear, and the open, straight ends of the spring will face to the front.

You will now insert the hammer into the lower receiver, on top of the trigger assembly. The legs of the hammer spring need to rest on the pin that is holding the trigger in place. Press the hammer down and towards the front of the receiver. Once aligned, you can insert the hammer pin through the receiver until it is flush on both receiver surfaces.

Don’t forget to protect your receiver frame from the hammer movement by using a hammer block before function testing. Ensure that the hammer releases forward when the trigger is pulled. With the trigger depressed, reset the hammer to check that the disconnector engages the hammer. This is also the perfect time to measure your AR-15’s trigger pull with a trigger pull gauge.

Selector and Pistol Grip Installation

Begin by lightly oiling the selector and applying grease to the detent and detent spring.
 Cock the hammer back, then insert the selector on the left side of the receiver.

You will find the selector detent hole on the bottom of the receiver, just below the selector. Insert the pointed end of the detent into the hole.

Turn to your pistol grip and you will note a small hole where the grip will match up with the receiver. Insert the detent spring into this hole in the grip. Next, place the pistol grip onto the receiver, aligning the detent spring in the grip with the detent just below the selector on the receiver. The spring will fit into this detent hole. Lastly, install the grip screw and washer by approaching through the bottom of the grip.

Test your selector by putting it in “safe” mode. The trigger should stay locked in place when you attempt to depress it. Once put into “fire” mode, you will be able to depress the trigger, sending the hammer forward. Again, be sure to protect your receiver frame from hammer movement by using a hammer block.

Buffer Assembly

First, clean any debris from the threads of the buffer tube. Grease the retainer spring, retainer, buffer and buffer spring.

With the notches of the castle nut facing the end of the buffer tube, thread the castle nut onto the buffer tube until it is tight. With the raised portion of the receiver end plate facing away from the buffer tube, place the receiver end plate onto the end of the buffer tube. 

Apply a dab of Loctite 242 to the threads of the buffer tube and the inner thread of the receiver. You are now ready to twist the buffer tube onto the receiver, but do so only far enough to prevent the buffer tube from falling off. 

Once your buffer tube is in place, you will notice a small hole in the receiver lying just in front of the end of the buffer tube. Place the buffer retainer spring into this hole, followed by the thick end of the retainer. While depressing the retainer with a punch, resume screwing the buffer tube until it holds the edge of the retainer down. The tip of the retainer remains exposed, and should easily move up and down. Tighten the tube with an AR-15 combo wrench.

Next, slide the buffer spring into the buffer tube, followed by the pointed end of the buffer. You will need to depress the retainer in order to slide the buffer fully into place. The tip of the retainer will then hold the buffer inside the tube. Ensure that the buffer moves freely in this space.

Buttstock Assembly and Takedown Pin Installation

After applying a dab of oil to the takedown pin, insert it into the takedown pin hole on the right side of the back of the receiver.

On the back of the receiver, just below and to the right of the area where the receiver meets the buffer tube, you will find a small hole. Insert the takedown pin detent into this hole, followed by the takedown pin detent spring. With a portion of the spring still exposed, carefully slide the receiver end plate forward and depress the spring straight into the hole until the end plate is against the receiver. You can then continue threading the castle nut until it holds the end plate firmly in place against the receiver, then snug it with a wrench or screwdriver. Lastly, ensure the detent has fallen into the groove in the takedown pin by rotating the takedown pin.

Congratulations! You have now successfully built your AR-15 lower receiver. Happy gun building!