How To: Build an AR Lower Receiver
LEGAL NOTICE: This piece is intended for the law abiding public, where allowed, to purchase and assemble a working AR-15 Lower. This piece will not address any conversions, nor will it address any working parts of an M4, or fully automatic, lower receiver. Neither the authors, nor this website, make any representations about the laws in your specific area. You need to do the research and see if you can legally purchase and own said weapon. Neither the authors, nor this website, make any guarantees about the performance of said product, and any fine tuning or adjustments that you feel are needed, or are just plain curious about, should be done by a licensed and trained gunsmith. Precede at your own risk gentlemen, this is meant as an instructional guide only for what you would like to do.
For this thread we will be taking the lower receiver of an AR-15 from a stripped down state, and using the parts available from various on line sellers, to LEGALLY assemble them into a working model. It should be noted that frequently we will be referring to the parts by their initial product names from the first manufacturer, Colt Firearms. Where relevant, we will also try to give a more common recognized name for the same pieces so you will have two names to shop and acquire with.
The immediate concern is that with the pending Presidential change, in one shape or form, that another “Assault Weapons Ban” will come into effect, and with the Left in charge of the House and Congress, will be passed. To beat this possible issue to the punch, a concerned person might want to get a lower receiver of an AR-15 “on the books” so they will be grandfathered in, should such a new law come into effect.
Let’s be honest, not everyone can pony up $800 or $1000 or $1500 for a top of the line AR on a couple of weeks notice. If you can, this thread isn’t for you. But if you can’t, you might want to purchase a lower receiver, which is the only piece of the weapon that requires the BATFE/FBI background check. Once you legally own a lower receiver, you can assemble the weapon at any point over time and have a legal AR-15 that is not in violation of any pending laws.
The first picture shows a list of tools, that while not completely NECESSARY for the two part project we will be undertaking in the coming month(s); it sure will make the job a lot easier. There are two wrenches ( an Armorer’s wrench and an M4 Stock Wrench ) and there are two parts used to hold the upper and lower receivers ( Upper Receiver Vise Block and Lower Receiver Vise Block respectively ) while complex work is being done.
These tools are available from Brownell’s or Midway along with several other online retailers. While you don’t “NEED” all of them for this part of the project, the M4 Stock Wrench would be the only tool that is highly recommended by the combined authors.
Here are the parts, and their names, that we will be dealing with in this installation procedure.
The next picture is where I like to start when assembling a new / used lower receiver for reasons that will become apparent as we move along.
On the model in question I have an Olympic Arms lower receiver and a Hogue pistol grip that we will be attaching together. As you can see in the picture, there are ( 2 ) important parts that will need to be in place prior to tightening the basic screw located inside the pistol grip. In addition the Fire Control Selector, which as the name applies allows you to switch from “SAFE” to “FIRE” will need to be slipped into place before proceeding.
Slide the Fire Control Selector into the lower receiver from the left to right. It will swivel loosely at first and it really doesn’t make any difference if you place it in the “FIRE” or the “SAFE” position. It is important that the selector points definitively towards one or the other.
On the underside of the receiver you will see an angled hole that will house both the Fire Control Selector Detent and Fire Control Selector Spring. Following the site path of the machined hole, you will be able to see that the Fire Control Detent will need to slip inside this machined hole first. It will, effectively, provide the positive “ON” or “OFF” as you rotate from “FIRE” to “SAFE”. Once the Detent Pin is in place, you will need to back this up with the Fire Control Selector Spring ( see picture ). Once the pistol grip is screwed in place, the Fire Control Selector Spring will provide the tension on the Fire Control Selector Detent which will lock the Fire Control Selector in place.
Congratulations! You should now be able to hold your new lower and, with your thumb, flick your Fire Selector from “FIRE” to “SAFE” with a definitive, positive “Click” with each change. The first stage of this installation is now completed.
The next step will be to install your new trigger. On the display model I have a Rock River Arms 2 Stage Trigger that we feel is the best trigger for tactical operations right now. Feel free to purchase whatever trigger you want, but this install will be for 2-stage triggers and won’t include pictures for the additional Disconnector and Disconnector Spring associated with G.I. Triggers as I don’t have access to one currently.
The Trigger Spring is going to provide some resistance as you push the Trigger into place to be able to insert the Trigger Pin. This is good and means that you have the Trigger Spring in the right direction. You will notice that you have to slip the curved part of the Trigger through the hole in your receiver, and then slip the back of the Trigger under the Fire Control Selector that is already in place. This makes it easier, for me, to keep the Trigger in place to insert the Trigger Pin because it is not looking to “jump” out of the lower. The next picture illustrates a pin punch being used to hold the Trigger in place prior to installing the Trigger Pin. The picture after that is with the Trigger Pin about to be put in place that will render the Trigger secure.
Install the Trigger Pin from Left to Right on the Receiver. The location of the grooves on the Trigger Pin does not make a difference. Once the Trigger Pin is in place, you will be ready to install the Hammer and Hammer Spring.
In this picture you can see that both the Fire Control Selector and the Trigger have been pinned in place. In the picture you can see the Hammer, equipped with the Hammer Spring, ready to slide into the Lower Receiver and be pinned in place. Notice that the long “legs” of the Hammer Spring are down and will go OVER the Trigger Pin housing. This will result in considerable upward and outward force on the Hammer as you try to pin it in place. This is normal and what you want to have happen if you have the Hammer Spring in the right alignment.
Insert your Hammer Pin, once again Left to Right, as you did with the Trigger Pin. As before, it makes no difference which direction the Hammer Pin itself goes into the Receiver, as long as it goes all the way through and pins the Hammer tightly in place. Upon completion, your Receiver should look something like this:
You will be tempted to cock the Hammer on your AR Receiver and “Fire” it. DO NOT DO THIS!!!. You risk damaging both your Hammer and your Receiver. If you want to cock the Hammer into the down position and check your Fire Control Selector in the “SAFE” position, it should work perfectly at this time, make sure that you have your off hand in place to stop the Hammer from slamming into your Receiver, just in case. The “SAFE” mode should work, keeping your Hammer from moving from the cocked position. The “FIRE” mode of the Fire Control Selector should work as well, so if you select “FIRE” and pull the trigger, your hammer should spring forward, so make sure your hand is in place to stop its forward momentum from slamming into your receiver and risking damage.
If your Hammer locks in the down position, doesn’t move on “SAFE” but springs forward on “FIRE” when the trigger is pulled, Congratulations! You have successfully completed half the Lower Receiver Assembly.
The next step will be to install the buttstock/buffer tube. In these pictures I have included a 6-position adjustable stock ( Adjustable Recoil Extension according to Colt ) that I got for my AR from Midway USA. This model has served me well thus far and has a wide range of adjustments for CQB to Defensive Rifleman positions. If you purchase another buttstock, the installation will be very similar, but always read the damn directions; that is why they are included.
Here is a picture to show you the first step in prepping your adjustable stock. You will need to take the aluminum Recoil Buffer and slide it inside the Recoil Buffer Spring. These two items will eventually slide, Spring first, into the Recoil Tube ( Receiver Extension according to Colt ).
Next you will need to take the Receiver Extension Nut ( Colt ) also known as the Buffer Tube Lock Nut and thread it on to the stock in question. Make sure and thread it a ways onto the stock as you will need the extra threads for the final installation. There are notches cut out on the Receiver Extension Nut ( Colt ) / Buffer Tube Lock Nut. The notches on the Nut go towards the back of the Receiver Extension and away from, for example, the handle and Lower Receiver.
The next thing is to place the Receiver End Plate ( Colt ) AKA Alignment Washer AKA Buffer Washer onto your Adjustable Recoil Extension /Buffer Tube Extension /Stock. There is a notch on the End Plate, dead center in the bottom of the circle. You will notice there is also a notch on the Extension Tube / Buffer Tube Extension. Align them with the bump on the washer towards the front, towards the lower receiver, and make sure you have plenty of threads left to insert this whole unit into the lower receiver.
This picture shows where the Buffer and Buffer Spring will go once you have the other parts in place. You will not need to have them in position for the next step of the operation, this was instructional for the assembly of the Adjustable Recoil Extension / Buffer Tube Extension / Stock only. With the nut and washer in place, and plenty of thread left, you are ready to prep the Receiver for the attachment of your Stock.
In this picture it shows the order that the Take Down Pin, the Take Down Detent and the Take Down Spring will come together. The Receiver End Plate ( Colt ) AKA Alignment Washer is shown for reference.
This is a picture from the back of the lower receiver. In it, you can see that I have put the Take Down Pin in place and that the Take Down Spring is holding the Take Down Detent in place pending the attachment of the stock. Prior to taking the next step however, you will want to insert both the Recoil Buffer Detent Pin and Recoil Buffer Detent Pin Spring into the hole in the threads. I won’t kid you; this pin is one of the two real bastards on this assembly. You have to compress it with your finger while threading your stock into the receiver to a point where the threads on the stock just barely hold the outer lip of Detent down, but allow the point on the Detent to be up high enough to hold the Buffer and Buffer Spring in place inside your stock.
Now it’s time to put in the last part on your new AR Lower. You could do this step earlier if you wanted, but I was working in somewhat of a systematic format around the lower. In any event, you are now ready to place the Pivot Pin in place. Here are the three parts in question:
Remember when I said there were two truly bastard parts to install on this project. This is the other one. You have to place the Pivot Pin Spring into the AR Lower first. This is contrary to how you did it with the Take Down Pin Spring, but it will serve the same purpose, as you will soon see. The Pivot Pin Spring is large enough, that when you try to push the Pivot Pin Detent Pin into place, it’s going to try and jump right back out on you. The first time I ever took one of these apart, the Spring and the Detent jumped across the shop and my ‘Smith just laughed and said “What did we learn?” What I learned is that this pin Spring and Detent is a P.I.T.A.
Here is the Pivot Pin Spring and the Pivot Pin Detent in the Lower Receiver with a punch holding them in place. In this application the punch will be replaced with the Pivot Pin. Notice that the Pivot Pin has a groove down one side of the pin. You will need to align this slot with the Pivot Pin Detent so the Pivot Pin will stay firmly attached to the Lower Receiver.
In this last picture you can see that the Pivot Pin is in place and the Pivot Pin Spring and Detent will provide a positive start and stop to the travel of the pin when assembling or taking down your future AR.
Well, that is it for the AR Lower. Pretty simple, huh? If you have any questions, please feel free to PM me and I will see if I can’t point you in the right direction.
As always, this work was not a sole effort and I could not have produced such an in depth process report without the help and knowledge of Brett Evans of N.W. Armswerkes. His skill and knowledge is invaluable to me and I truly consider him a master of the modern firearm. In addition, I would like to thank the website of [ http://wigflip.com/roflbot/ ] as they have a valuable picture text tool that is easy to use, and most of all, free to the masses.
That's Awesome, JD.
Did you find it easy to follow? Could you build one yourself now if you had the parts?
Great writeup JD! I think I have a new summer project.
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