Step by Step: How to Assemble an AR-15 Lower Receiver.

This is a step by step instructional post about how to assemble an AR-15 Lower Receiver. Building your own AR-15 is not as hard as you think, it fun, and really allows you to personalize it as you go.

Take your time, go in order. I’ve included simple instructions and photos of every step to assist you in your build. This build has a few upgrades, like an ambidextrous safety switch but the steps are the same.

This is for informational and educational purposes. Anyone with basic tools and experience should be able to assemble a lower, but if you have any doubt, leave it to an experienced gunsmith.

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Tool List

Here’s a list of the basic tools required for this build.

Parts Required

AR-15 Lower Receiver Parts
AR-15 Lower Receiver Parts & Hardware

There are all kinds of options out there for part when it comes to building an AR-15. Here’s the basics for getting it together in a cost-effective, high-quality build. Click on the following to see pricing and/or order.

Magazine Catch Installation

Ar-15 Magazine Catch
Magazine Catch Parts

Now I will walk you through the assembly process. This is the easiest order of completion and the way I originally learned it. The magazine catch is step one. Find these three parts in your build kit: The button, the L-Shaped catch, and the magazine spring. This will be the largest width spring in the build kit.

Insert Spring for magazine catch
Insert Magazine Catch Spring

First, insert the spring into the magazine release button hole. The hole is oval, but there is a round recession in it for the spring.

Magazine catch button
Magaine Release Button

Set the magazine release button on the spring with the knurled side out. Most buttons that come with a build lit have the threaded hole all the way through, so be sure you have it facing the right direction.

ar-15 magazine catch
Press hard, go slow!

Apply pressure to the button, and hold it in place.

AR-15 Lower Receiver

While holding the button in, flip the receiver, and gently start threading the magazine catch through the body and into the button. You will need to keep the button pressed flush to start the threads.

Desert Guardian Lower Receiver Ar-15 Build
Insert Magazine Catch
Desert Guardian Lower Receiver Ar-15 Build
Gently thread the catch into the button.

GENTLY thread the catch into the button. Go slow, because as you get close, you will have to push the button in beyond the flush plane of the receiver wall to get it in position. Be careful not to scratch the receiver!

No gap

When the latch is threaded in enough, and the button is pressed, there should be no gap between the latch and the receiver as seen here.

Desert Guardian Lower Receiver Ar-15 Build

Mag Catch and Mag Release Button are installed!

Trigger Guard Installation

Trigger guard ar-15
Magpul MOE Trigger Guard and Hardware

Let’s get the Trigger Guard installed! Locate the guard and associated hardware. If your lower receiver does not have a permanent trigger guard as part of its machining, you will need to install one. Some have roll pins, others have spring-loaded pins already mounted, and the Magpul MOE had a roll pin and a threaded Allen screw. I chose the Magpul MOE because it has a slight bend to it and allows a little more room for shooting with gloves or if you have bigger fingers.

insert pin

First install the front Allen screw. Very simple.

roll pin installation

The rear uses a larger roll pin, which can be tricky. Get the roll pin set with a light tap from a hammer, then tap it in with the back “ear” supported. Go slow, and make sure you are lined up. Going too fast and misalignment can result in a bent “ear” on the down side. Another method to install this roll pin is to use a C-Clamp and slowly work it into place.

brass punch for roll pin

Once the roll in is in, use a brass punch to flush it up, or counter sink it just a couple millimeters.

Desert Guardian Lower Receiver Ar-15 Build
Trigger Guard Installed!

The Trigger Guard is installed! On to the next step!

Bolt Catch Installation

ar-15 bolt catch install assembly
Bolt Catch Parts

Next, locate the bolt catch, plunger, spring and roll pin. This is the small roll pin in the kit…smaller than the trigger guard pin. The spring is similar in size to the disconnect spring, but both ends are the same size. The disconnect spring has a wider opening at one end.

Desert Guardian Lower Receiver Ar-15 Build assembly

Put the plunger into one end of the spring, then drop the spring into the hole shown here.

Desert Guardian Lower Receiver Ar-15 Build assembly

The plunger will protrude just a little bit.

bolt catch

Slide the bolt catch into the slot on the top of the receiver. Put some pressure on the spring and line up the holes on the side for the roll pin.

bolt catch pin roll pin

The bolt catch roll pin can be tricky. I will set it in place and gently tap it with the plastic end of the hammer just to barely get it started. Then I use my Knipex channel locks to squeeze it slowly into place. I use liberal amounts of masking tape on the vise grips to avoid scratching the finish.

My advice here is GO SLOW. Take your time, really study the situation, and slide it into place. This is one of the few parts of assembly that feels like it takes three hands, but can be done easily of you take your time.

install roll pin ar-15 bolt catch

Gently squeeze, sliding the roll pin into place, being very aware if the alignment as you go.

Desert Guardian Lower Receiver Ar-15 Build assembly

Bolt Catch in in. Time to move on to the Fire Control System.

Trigger & Hammer Installation

ar-15 trigger and hammer assembly

Now we are going to install the Fire Control System. This consists of the trigger, Hammer, disconnect, springs, and pins. Get these parts together and note the wider opening at one end of the disconnect spring.

ar1-15 trigger and spring

Install the trigger spring like so. it can be a little tricky, so make sure it looks like this when installed.

ar-15 hammer and spring

Install the hammer spring. Note the different shape from the trigger spring, and how it is installed on the hammer.

fat side down ar-15 disconnector spring

The disconnect spring will snap into the round recess on the back of the trigger shown here. Insert it with the WIDE end into the hole.

ar-15 disconnector assembly

The disconnect drops into the slot on the back of the trigger as pictured here. It’s just resting there, so keep a finger on it.

ar-15 trigger assembly install

The assembled trigger then drops into the receiver and aligns with the rear pin hole.

correct trigger install ar-15

It should look like this from above.

insert pins ar-15

Depress the trigger assembly with your finger to compress the disconnect and line up the holes. Take the pin and push it through the trigger and disconnect, and flush with the other side. It should slide easily with the holes lined up, but if the spring tension puts resistance on it, gently tap it with the plastic end of the hammer until flush. DO NOT pound on it…it should go in easy. If it does not, you are out of alignment.

trigger pin flush ar-15
Trigger pin installed

Here is the pin flush and installed correctly.

ar-15 hammer and spring assembly

The hammer and spring are next. The two spring “spines” go to the rear on either side of the trigger assembly, and it will take a little pushing against the spring tension to put the hammer in.

insert hammer in with punch if needed

Using your finger, keep pressure on the hammer and align the holes. You may notice the pin going a bit sideways, so I got a tip once to cock the hammer back to help with alignment of the holes. Give it a little wiggle, watch the holes, and slide the pin flush. Give it a gentle tap if needed.

Desert Guardian Lower Receiver Ar-15 Build assembly
Fire Control System Installed

The Fire Control System is now installed. Just a few steps left.

Safety, Takedown Pins, Grip & Buffer Tube Installation

safety takedown pins grip and buffer tube ar-15 parts installation

This seems like a lot of steps at once, but these parts all work in conjunction with each other. Locate the parts shown here. NOTE: one skinny long spring and one small detent are missing from the picture. (there will be two of each in your build kit)

There should be THREE detents, two small and one large, and THREE detent springs, two longer and skinny, and one slightly shorter and fatter.

Desert Guardian Lower Receiver Ar-15 Build assembly

Start by cocking the hammer back. This makes it easier to seat the safety.

ar-15 safety installation

Insert the safety selector into the safety hole.

ar-15 safety assembly

Set the selector to the fire position (up).

safety detent spring grip ar-15

Now it’s getting to a spot where it will feel like you need a third hand again, but go slow, be aware of gravity, and it will all come together. Holding the safety with your finger, flip the receiver upside-down and insert the large detent in the hole shown here.

grip safety detent spring ar-15

While holding the receiver and keeping the detent and safety from falling out, insert the slightly fatter and shorter detent spring (the one that doesn’t look like the other two) into the hole on the grip shown here. Remember, go slow and everything will come together.

Desert Guardian Lower Receiver Ar-15 Build assembly

Still holding the safety selector, slide the grip onto the receiver. Line up the spring and the detent. I find if you hold everything sideways at this point, gravity doesn’t make things shift and fall out. The grip will most likely slide on very tight, so you may need to wedge it on and tap it with the plastic side of the hammer.

grip screw ar-15
Grips Screw

Once the grips is in place, insert the grip screw.

Desert Guardian Lower Receiver Ar-15 Build assembly

If the grip isn’t completely seated, you can draw it in tight with the screw. Just make sure the spring and detent are aligned.

ar-15 takedown pins assembly installation detent spring

Let’s move on to the rear takedown pin. You will need one long spring, one detent and the shorter pin indicated here.

Desert Guardian Lower Receiver Ar-15 Build assembly

Insert the rear takedown pin into the right side of the receiver as shown, with the channel facing the rear detent hole.

rear detent ar-15

Drop in the detent. It should sit in the channel on the takedown pin.

rear detent spring ar-15

After you drop in the detent, insert a detent spring into the hole.

buffer retainer spring install

While holding the detent spring in place, insert the buffer retainer spring. his is the largest diameter spring remaining in the kit.

buffer retainer ar-15 spring

Drop the buffer retainer over the spring, and set the receiver gently aside so none of the springs fall out.

buffer tube ar-15
Buffer Tube and Hardware

Locate the buffer tube, castle nut and end plate shown here.

buffer tube castle nut ar-15

Thread on the castle nut as shown with the square cutouts toward the rear,

buffer tube end plate ar-15

Slide on the end plate. Pay attention to TWO things: Note the channel it sits in on the buffer tube, and note the protrusion on the plate. It points TOWARD the receiver. Do not get this backward.

buffer tube installation

Pick up the receiver with your thumb over the buffer retainer. Thread the buffer tube on slowly and watch as it approaches the buffer retainer and detent spring. The end plate will hold the detent spring in place.

Desert Guardian Lower Receiver Ar-15 Build assembly

As the tube approaches the retainer, it will cover the lip on the side. NOTE: if it’s covered, and you make an adjustment, the spring and retainer can fly out and disappear forever. Trust me, I’ve done it.

Desert Guardian Lower Receiver Ar-15 Build assembly

Once the lip of the retainer is covered, make sure the end plate is in the correct alignment and the detent spring is straight. Sometime this takes a little manipulating, but it will go.

Desert Guardian Lower Receiver Ar-15 Build assembly
Proper Buffer Tube Installation

The proper installation of the buffer tube will cover the buffer retainer like this.

Desert Guardian Lower Receiver Ar-15 Build assembly

With the buffer tube threaded to the correct depth and orientation, thread the castle nut snugly against the end plate, holding everything in place.

Desert Guardian Lower Receiver Ar-15 Build assembly

Using an armorers tool or a brass punch, tighten the castle nut against the backplate. Your buffer tube is now secure.

front takedown pin ar-15 detent spring
Front Takedown Pin Parts

The front takedown pin is next. This requires the larger takedown pin, and the other skinny spring and detent shown above.

detent spring ar-15

Slide the detent spring into the hole on the front right of the receiver shown here.

detent pin ar-15

The detent goes in next. This is another tricky part. The spring is strong, and the detent needs to be held flush to install the pin.

Desert Guardian Lower Receiver Ar-15 Build assembly

Using a set of needle nose pliers, firmly grasp the detent and slide it as far as you can in the hole.

front takedown pin ar-15

This picture is a little blurry, sorry. I used a credit card to hold the detent flush, then squeezed the takedown pin into position. The detent locks into the channel on the takedown pin. The pin clicks into place, and your are good to go!

Buffer and Stock Installation

buffer and stock assembly installation ar-15
Buffer & Stock

Finally, grab the buffer spring, buffer and stock and lets get them on!

buffer spring

Slide the buffer spring into the buffer tube.


The buffer should click in when it depresses the buffer retainer on the way by. The retainer holds the buffer in place.

adjustable stock

Slide the stock onto the buffer tube.

Desert Guardian Lower Receiver Ar-15 Build assembly

This buffer tube is a standard 4 position mil-spec tube, and the stock has a spring loaded button to release the lock and slide to a different position. To lock it on, pull down on the little knobs here, and slide it on. Most standard stocks have a spring loaded lever that need to be pulled and sthe stock slipped on. Then you just manipulate the lever and chose your desired stock position.

Congratulations! You’re Finished!

completed ar-15 lower receiver build. Desert Guardian Lower Receiver Ar-15 Build assembly

Congratulations! You’ve just completed a crash course in building your own AR-15 Lower Receiver. Building your own AR-15 is fun and rewarding. There’s nothing better than shooting and enjoying something you assembled with your own hands.

Questions? Comments? Ideas for future projects? Comment below!

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