It’s one of the most customizable and easy-to-build weapons on Earth. It’s also the most popular firearm bought and sold in America. The AR-15 is an incredible weapon. You’re just as likely to put together your own black rifle (or pistol) as you are to simply buy one. This guide covers the bulk of the assembly work involved with building your own AR: Installing the lower parts kit. The kit we’re using is the Anderson Stainless LPK. It’s a beautiful, crisp, reliable (and affordable) trigger that’ll we have a review out for soon.

Tools Required

  • Armorer’s Wrench: For tightening the buffer tube’s castle nut.
  • Lubricant or Oil: For assisting with pin and spring installations.
  • Masking Tape: For covering your receiver to prevent scratches.
  • Punches: 3/32″ and 5/32″ punches are required for the roll pins.
  • Gunsmithing Hammer: For tapping your punches and roll pins safely.
  • Blade or Small Knife: Depresses a spring-loaded detent to install the front pivot pin.

Parts Required

  • Lower Parts Kit: Ensure the kit includes the hammer, trigger, disconnector, and all the required parts before you start the install.
  • Buffer Tube: Ensure your buffer tube’s threads match the threads on your castle nut and lower receiver’s buffer tube housing.
  • Castle Nut: Most lower parts kits don’t include a castle nut, though your buffer tube usually includes one. Make sure you have one.
  • Latch Plate: This plate is the same story as the castle nut. Make sure you’ve got one. If not, try grabbing one with a sling mount.
  • Stripped Lower Receiver: For this install, we’re using a mil-spec forged receiver. These instructions work for billet lowers, too.

Step 1: Assemble Trigger and Trigger Spring

Install the trigger spring onto the trigger. The loops of the spring should rest around the trigger pin housing. Make sure the tabs of the spring face in front of the trigger.

Step 2: Assemble Hammer and Hammer Spring

Assemble the hammer and hammer spring in the same fashion as the trigger. Ensure both loops for the spring rest around the hammer pin housing. Make sure the ends of the face point forward of and down from the face of the hammer.

Step 3: Install Disconnector Spring on Trigger

The disconnector spring is shaped like a cone, with a skinny end and a fat end. Ensure the fat end of the spring rests inside the top of the trigger.

The spring can be a tight fit. It helps to use the 3/32″ punch to gently press the bottom of the spring to get it fully seated.

Step 4: Install The Disconnector and Trigger

Rest the disconnector atop the trigger, ensuring the notch in the bottom of the disconnector rests atop the disconnector spring.

Drop the disconnector and trigger into the receiver. You’ll need to apply gentle pressure to the assembly to line all the holes up for the trigger pin install.

It helps to lubricate the trigger pin hole, too. Collect the trigger pin and insert it from the left side of the receiver with the un-notched side first. Gently tap on the pin with your gunsmith hammer while applying pressure to the disconnector and trigger to get the pin seated. Once installed, the pin should be flush on both sides of the receiver.

Step 5: Install The Hammer

Drop the hammer into the receiver oriented as shown. The hammer spring is much stiffer than the trigger spring, so it will take considerably more force to press the hammer into place and align the holes for the hammer pin. When dropping the hammer into the receiver, the ends of the hammer spring should rest atop the round trigger pin housing on the trigger itself, facing the rear of the receiver.

Repeat the installation steps for the hammer pin as you did the trigger pin: Orient it so the un-notched side goes in first, and lubricate the hole to help with installation. The hammer pin should also be flush on both sides of the receiver.

Step 6: Check Hammer, Disconnnector, and Trigger Functions

The entire fire control assembly is now functional. To confirm its function, cock the hammer by pressing it down. It should click and remain in this downward position.

With the hammer held by your finger (so as not to crack the receiver), squeeze the trigger. The hammer should release to the upward position. Keep the trigger squeezed after releasing the hammer. Press the hammer back down. The hook on the disconnector should capture the hammer. Once it does, release the trigger. You should hear a “pop”, followed by the hammer being captured by the trigger as it resets.

If your disconnector, trigger, or hammer fail to function properly, refer to the bottom of this guide to troubleshoot the issues.

Step 7: Install Trigger Guard (If Applicable)

Your lower parts kit should include a trigger guard if your receiver does not have an integrated guard. Orient the guard as shown and press the guard’s spring-loaded pin into the holes on the trigger guard mount near the magazine well.

Align the other end of the trigger guard with the rear mount’s roll pin holes. It’s very important you support the bottom of the mount for the pin. Hammering the roll pin into the holes could crack the mounts otherwise. We used the rubber handle from our pliers. Once supported, hammer the roll pin into place using the 5/32″ punch and gunsmith hammer.

Once seated, the trigger guard’s roll pin should be just slightly recessed or flush on both sides. Collect the magazine catch, magazine release button, and spring for then next step.

Step 8: Install Magazine Catch and Release Button

Insert the magazine catch through the hole in the magazine well. The threads should protrude from the opposite side.

On the opposite side of the receiver, insert the bolt catch spring onto the threads. You’ll need to carefully thread the magazine release button onto the threads while compressing the spring. The spring will make the button cross-thread often, so take your time and lubricate the threads.

Thread the button onto the magazine catch post until it’s flush with the receiver’s exterior. Flip the receiver back over.

Press the magazine release button into the receiver as far as possible. The magazine catch will pop out of its channel. Rotate the catch clockwise three to four rotations, align it with the channel, and release the magazine release button.

The magazine release button should now be flush with, or resting just inside, the exterior of the receiver. Test the function of the catch and release by inserting and dropping an empty magazine. Having some wiggle in an inserted magazine is completely normal and will not inhibit the function or reliability of your AR-15 in any way.

Step 9: Install the Bolt Catch Assembly

The bolt catch assembly is secured to the receiver with a roll pin. Tapping this roll pin into place is difficult and the risk of scratching your receiver is high. It’s a good idea to tape the area surrounding the bolt catch roll pin holes with masking tape.

Now insert the bolt catch plunger’s spring into the bolt catch hole.

Then insert the plunger top the spring. It helps to apply some oil here; the plunger can make for a tight fit until it’s worked in.

Now comes the hard part: The roll pin needs to be seated with the bolt catch inserted. Minor pressure’s required to line up the holes because the spring needs to be compressed just slightly. This can be difficult to accomplish while trying to get the pin started. It helps to first partially tape the place into the first hole, then align the bolt catch over the plunger.

Once partially seated, the pin should not fall out and you can align the bolt catch. Apply gentle pressure with your thumb and continue tapping the roll pin through the housing and bolt catch itself.

Once tapped into place, the bolt catch roll pin should flushed or recessed slightly on either side.

Step 10: Install Front Pivot Pin

You’ll need a knife, small blade, or thin, flat and rigid tool (like a guitar pick or screwdriver) to help with this installation. First, insert the pivot pin’s detent spring into the small hole to the right of the pivot pin’s first housing.

The pivot pin detent will need to rest atop the spring inside this same hole. Getting the detent seated fully requires compressing the spring. At this point, the chances that pivot pin will fly out of its hole and into the carpet is high. Before getting the detent in the hole, grab a pair of pliers and that small blade or flat tool. Have the pivot pin handy, too.

Now compress the spring and insert the detent, using the blade or flat to keep the detent compressing the spring underneath. Grab the pivot pin and orient it so the flat side of the pivot pin head is oriented with the detent’s hole.

While keeping the detent compressed, slide the pivot pin into the first housing. A cut-out channel in the pin will lock with the detent, trapping it. Ensure the detent seats fully into both housings.

Step 11: Install Safety Lever and Pistol Grip

The safety lever must be installed with the pistol grip, as the grip serves to trap the lever in place via a detent and spring. Collect all components, including the grip’s washer and bolt. Make sure you have the correct Allen key, socket, or screwdriver for your grip’s bolt.

Insert the safety selector lever into the left side of the receiver. The lever makes for a tight fit, so it helps to lubricate the holes in the receiver.

Flip the receiver upside down and ensure the safety lever is either oriented fully in the “FIRE” or “SAFE” position. Drop the safety lever detent into the small hole to the right side of the pistol grip mount.

Grab the grip and insert the safety lever detent spring into the small hole drilled atop the pistol grip.

Slide the pistol grip onto the receiver’s grip mount while ensuring the safety lever detent’s spring is oriented with the detent hole.

Ensure the pistol grip hole is aligned with the threaded hole on the receiver, and drop the grip bolt and washer into place. Tighten with the appropriate tool. Be careful not to over-tighten. The aluminum receiver can crack with excess torque. The washer will prevent the grip from becoming loose when firing.

Step 12: Install Rear Takedown Pin and Buffer Tube

Insert the rear takedown pin into its hole on the right side of the receiver. Ensure the channel of the pin is facing the rear of the receiver. Insert the takedown pin detent into the small hole at the rear of the receiver, underneath the buffer tube housing.

Insert the rear takedown pin detent spring atop the detent in the same hole. Collect the buffer tube, castle nut, and latch place. Thread the castle nut onto the buffer thread’s with the “crown” of the nut facing the rear of the tube. Slide the latch plate over top the threads and castle nut, with the raised side of the plate facing the receiver.

Thread the buffer tube onto the receiver partially, until the latch plate begins to contact the rear takedown pin detent spring.

Step 13: Install Buffer Retainer

Locate the buffer retainer and spring. Drop the spring into the hole drilled into the bottom threads of the buffer tube housing.

Next, drop the buffer retainer atop the spring.

Now compress the buffer retainer down into the hole in the threads. It helps to have a large punch or tool to compress the retainer and spring. While compressing both, continue threading the buffer tube into its housing. Thread the buffer tube in until the lip of the tube meets the nipple on the buffer retainer. The tube should be resting against the nipple without applying pressure to it.

Step 14: Tighten the Buffer Tube

With the buffer tube seated, press the latch plate up against the backside of the receiver, ensuring it compresses the pivot pin detent spring into its hole.

Next, hand-tighten the castle nut against the latch plate to keep everything seated. Lastly, tighten the castle nut with an Armorer’s Wrench. Loctite can be used, though this is not typically necessary with appropriate torquing. The castle nut may also be staked in similar fashion to the gas key on a bolt carrier.

Installation Complete

With the buffer tube tightened, your installation is complete!

Troubleshooting Trigger and Reset Failures

To troubleshoot your trigger, it’s a good idea to perform a function check to help identify where the issue is occurring.

  • With the safety lever on “SAFE” and the hammer cocked, squeeze the trigger. The hammer should not fall nor should the trigger move.
  • Place the safety lever to “SEMI”. Pull the trigger while supporting the hammer with your finger. The hammer should fall. Continue holding the trigger to the rear and re-cock the hammer. Release the trigger slowly. You should hear a “pop” as the disconnector hands the hammer off to the sear on the trigger. The hammer should remain cocked.
  • Support the hammer and squeeze the trigger again. The hammer should fall. Re-cock the hammer and release the trigger. Place the selector lever back to “SAFE” and attempt to squeeze the trigger. The hammer should not fall.

Hammer Won’t Cock or Release

If your hammer fails to cock when pressed down into the receiver or fails to release when the trigger is squeezed while the safety lever is on “SEMI”, the trigger should be removed and the sear checked for manufacturing defects, cracks, burs, or rough edges.

Trigger Won’t Reset or “pop” When Squeezed then Released

For the trigger to reset, the disconnector needs to hand off the hammer to the sear on the trigger when it’s released. If the hammer doesn’t release from the sear when the trigger is released, this is known as a “trigger reset failure”. Check all the following:

  • Disconnector and hammer hooks: If the hooks on the disconnector or hammer are gritty or have rough edges or burs, the hammer may fail to slide off the disconnector as the trigger resets. These hooks should be inspected and very lightly polished as required. Over-polishing or removing too much material could cause the trigger to fail completely.
  • Hammer and trigger springs: Ensure both the hammer and trigger springs are oriented correctly, as shown in the install steps above. If a spring is installed upside down or facing the opposite direction, the trigger and hammer won’t function.
  • Check disconnector spring: If the disconnector spring was installed upside down on the trigger, the disconnector may not function. The fat end of the spring should be resting inside the trigger, and the skinny end interfacing with the disconnector’s notch.
  • Polish trigger/hammer pins: The trigger and hammer pins should be smooth, but they can be gritty or require very light polishing. Careful, removing too much material from either pin will cause the pin to slide out from the receiver when fired.

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