AR15 Lower Receiver Assembly



Before starting any work on any firearm be sure that the firearm is clear. Do not touch the trigger until the firearm has been cleared. Inspect the chamber to ensure that it is clear of ammunition and that there s no ammunition positioned to be chambered. Do not keep live ammunition near your workspace. Once you have checked the firearm, check it again!

Tactical Works, Inc. takes no responsibility for the accuracy and/or safety of this “how to”.  This post is simply for your reading enjoyment. Before shooting any firearm have it inspected by a qualified gun smith.

This post will cover the assembly of an AR15 lower receiver with a lower parts kit (LPK) and a 4- position CAR stock. The LPK contains many small parts including springs. During installation you will be compressing some of these springs and they have a tendency to “shoot” from the receiver when attempting to install. WEAR SAFETY GLASSES. You can’t shoot your rifle if you don’t have your eyes.

AR15 Lower Assembly

Parts Needed: AR15 lower Receiver, lower parts kit (LPK), AR15 stock, AR15 stock completion kit (receiver extension (buffer tube), Buffer Spring, Buffer, End Plate, and Castle nut) for adjustable stock.

Tools: Small Hammer, Pin Punch, Pivot pin installation tool, Slave Pin, painter’s tape, gun oil

Your first build is a little intimidating especially when you dump out the LPK bag. There seems to be 100’s of small parts and really no clue where to start. After a few builds will be fairly confident and should be able to assembly a lower receiver in around 30 minutes.

Where applicable we will discuss the “text-book” method and tools for assembling the lower receiver and parts kit. We will also share a few ways to assembly the lower if you do not have some of the specialized tools.

A great reference is the U.S. Marine Corps technical manual TM 05538C-23&P/2. Rifle, 5.56-MM, M16A2.

The forums are also a fantastic resource for all AR15 related questions.

STEP ONE: Front Pivot Pin Installation.

Parts:Front Pivot/Takedown pin, Takedown pin detent, and Helical detent spring.

Tools: Pivot Pin Installation Tool

This job can be done without the use of tools however proper tools make the job much easier.

First is to find the Detent Spring, Detent, and Pivot Pin.

Front Pivot Pin Parts

In your LPK there should be two Detent Springs, one for the front Pivot Pin, and on for the Rear Takedown Pin. They are both the same size so either spring will work HOWEVER, there is also a Selector Spring (Safe Fire) that is similar to the Detent Springs but is it shorter and is a little larger in diameter. The Selector spring should not fit into the Front Pivot Pin Detent hole so mixing the springs up is difficult.

Detent and Selector Spring

If you don’t have a Pivot Pin Installation tool you can use the Pivot Pin to compress the spring and detent into the detent hole.

Detent Spring Installed

Detent spring and detent installed

Here is where you need your safety glasses. If you don’t have a Pivot Pin Installtion tool you’ll insert the Detent Spring into the detent hole in the lower receiver. Place the detent on top of the dentent spring and use the Pivot Pin to compress the spring and detent into the receiver until the Pivot Pin will slide into the Pivot Pin hole in the lower receiver. Sounds easy doesn’t it.

Be careful the spring will be under a lot of pressure and can easily launch the little detent across the room.

If you can’t convince the little detent to go into it’s hole you can try inserting a  hex key (3/16) or a punch that is a little smaller in diameter than the Pivot Pin hole from the opposite side of the receiver. Three hands are a great help here. Get the detent started with some needle nose pliers and compress the detent and spring with a very thing piece of metal or strong plastic. I have successfully used a the tip of a butter knife. Make sure you tape all edges of your tools so you don’t scratch your new lower.  Once the spring and detent are compressed enough to push the Hex key through the Pivot Pin hole push the hex key through. This should hold your detent and spring in place.

Next take your Pivot pin and slowly push the Pivot Pin into the Pivot Pin hole while pushing the hex key out. Again, sounds easy. Stick with it and you’ll get it.

If you’re lucky enough to have a Pivot Pin Installation tool then you’ll simply insert the Pivot Pin Installation tool into the oppisite side of the lower receiver until the hole int the tool lines up whit the Detent hole in the receiver. Drop the detent spring through the Pivot Pin Installation tool and then drop the detent through the Pivot Pin Installation tool. Use a 3/32 punch to compress the detent and the detent spring until you can turn the Pivot Pin Installation tool 90 degrees to lock the detent and spring in the compressed position. Take the Pivot Pin and slowly push the Pivot Pin into the lower recevier while pushing the Pivot Pin Installation tool out of the lower recevier.

Pivot Pin Installed

The hardest part in now over. Time to have some fun.

Step Two: Magazine Catch

Parts: Magazine Catch, Magazine Button, and Magazine Button.

Tools: None

Gather the Magazine catch parts.

Magazine Catch Parts

Insert the Magazine Catch into the lower receiver from the left side. This will be the side with the serial number. Once the Magazine Catch is in place slide the Magazine Spring over the threaded post on the Magazine Catch from the right side of the lower receiver. This is the side without the serial number.

Magazine Spring Installed

With the Magazine Catch and Magazine Spring in place hold the the Magazine Catch firm and thread the Magazine Button onto the threaded post of the Magazine Catch. Righty tighty.

After you have a few threads on you’ll want to push on the Magazine Button and screw the Magazine Catch into the button. The goal is to get the Magazine Catch close to flush with the Magazine Button. It might be necessary to push the Magazine Button into the lower receiver further than you can with your fingers to get the Magazine Catch flush with the Magazine Button. In addition, you’ll want to make sure that you don’t scratch the receiver with the Magazine Catch as it tightens.  Use a small punch give yourself a little more clearance but make sure you tape any tools that might scratch the Magazine Button.

Button Finished

When the post is flush with the button you’re done with this step.

Step Three: Bolt Catch

Parts: Bolt Catch, Bolt Catch Plunger, Bolt Catch Spring, and Bolt Catch Spring/roll pin.

Tools: 3/32 Punch, Small Hammer, Tape, Vice Grip/Pliers.

The use of tape is strongly suggested for this step. You’ll be driving the Bolt Catch Spring, also known as a roll pin, into the lower receiver.  There are two ways that you can install the Bolt Catch roll pin. First, you can use a hammer and a punch to start the roll pin, align the Bolt Catch, and then drive the roll pin home with a small hammer or, what many builders do, is start the roll pin with a hammer and a punch and then use pliers or vice grip pliers to “press” the roll pin into the lower receiver.

It also never hurts to apply a thin coat of gun oil on the roll pin before installing.

Roll Pin Started

Above the roll pin is started but does not enter the space where the Bolt Catch will be installed. The roll pin is started to make it possible to assemble the lower receiver with only two hands.

Once the roll pin is started put the Bolt Catch Spring onto the narrow end of the Bolt Catch Plunger.

Bolt Catch Plunger and Spring Installed

Drop the Bolt Catch Spring and Bolt Catch Plunger into the lower receiver. Spring goes into the receiver first and then the Bolt Catch Plunger. Remember the narrow end of the Bolt Catch Plunger goes inside of the Bolt Catch Spring. spring and plunger installed

Position the Bolt Catch in the lower receiver and grab your pliers.

You will need to press the bolt catch into the lower receiver while “pressing” the roll pin into the lower receiver with the pliers. You may need to work the Bolt catch in and out a little until the roll pin find the hole in the Bolt Catch.

Pressing in the roll pin

(Add more to using a punch instead of the pliers)

If you are using a punch to do the job. (ADD MORE IN THIS AREA)

Bolt Catch Done

Remove the tape and you’re done with this step.

Step Four: Trigger Guard Installtion

Parts:  Trigger Guard and Trigger Guard Spring/Roll Pin.

Tools: 1/8″ Punch or Taped Pliers/Vice Grips.

Trigger Guard Parts

The installation of the standard Trigger Guard is one of the easier tasks when building your lower however its very important to take care because it is easy to snap off the lower receiver extension or “ears” on the lower receiver if you do not support the receiver when driving the roll pin into the lower receiver.

One side of the Trigger Guard will have a pre-installed detent on the front end of the trigger guard. There will only be one hole in the receiver extension toward the front of the receiver. Compress the pre-installed detent and push the Trigger Guard inetween the two front receiver extensions until the detent “pops” into place.

Trigger Guard Installed front receiver extensions

Next, support the backside receiver extension with a block of wood or your work bench and drive the spring/roll pin through the top receiver extension and trigger guard until it is seated into the top and bottom receiver extensions. A little gun oil on the spring/roll pin will help with the installation.

Receiver Supported

Not all spring/roll pins are perfect and some may be too large to drive into the receiver extensions with out pressing or modification to the pin. If this seems to be the case do not force it. It is much easier and less expensive to find a new spring/roll pin than to replace your new lower receiver so again, pay special attention to putting too much stress on the receiver extensions/ears.

If you choose to user taped pliers here again do with care. Get the spring/roll pin started with a hand hammer and then using the pliers/vice grip “press” the pin into the top receiver extension, through the Trigger Guard, and into the lower receiver extension.

pressing the trigger guard into place

Trigger guard installation is done.

Step Five: Fire Control Group (FCG) installation.

Parts: Trigger assembly, Semiautomatic Disconnector, Disconnector spring, and the Trigger spring.


Slave PinFire Control Group Assembly

Assemble the trigger, trigger spring, and disconnector spring.

Trigger assembly

Once the trigger spring is installed on the trigger install the disconnector spring into the round cutout in the back of the trigger.

FCG assembled

You can place the trigger assembly into the lower receiver and then install the disconnector or you can install the disconnector and then install then entire assembly into the lower receiver.

To install the disconnector place the disconnector on top of the trigger so that the notch on the disconnector goes over the the disconnector spring.

disconnector installed

Drop the trigger assembly  into the lower receiver and align the the hole in the trigger and disconnector with the hole in the lower reciever. When the holes are alligned slide the trigger pin into the lower receiver, through the trigger, through the discconector, and flush with the back side of the receiver.

Installing the trigger pin

Trigger pin installed

Next install the Hammer, hammer spring, and hammer pin.

hammer parts

Take special care in the way the hammer spring is installed on the hammer. The “legs” of the hammer spring will lay on top of the trigger.correct hammer spring installation

Push the hammer into the lower receiver, align the hole in the hammer with the hammer hole in the lower receiver. This is a little tough because the hammer will be under some tension due to the spring.

Once the hammer is aligned with the lower receiver hammer hole slide the hammer pin through the receiver and hamer until flush.

You may need to tap on the hammer pin with a small hammer and punch to drive it home.  If you need to use tools to get the hammer pin in remember to take care and use tape on all tools to keep from scratching the lower receiver and its parts.

installing the hammer pin