How To Build An AR-15 Lower Receiver

I have a confession to make. I’m old. I have socks older than most of you. Those of you who have read much of what I’ve written for various gun sites may know that. No crime, being old, but it can color your thinking. I also have never been in the military, something I regret to this day. Therefore, I was not a user of the “black rifle” platform until fairly recent years. But – I got with the program, and own 4 at the present. I sure like these black guns now! 

(And, a side note… for those out there who think that “AR” equals only “.223/5.56”, guess again. Only one of mine is a .223. The other three are in calibers 7.62x39, .45ACP, and 9mm. More calibers are available in the AR platform than you could shake a stick at, if – as Groucho said – that’s your idea of a good time!).

My first AR was a Del-Ton Dissapator kit, provided for me by my good buddy Duane from his gun shop. This was several years ago when you just couldn’t find complete rifles of the AR persuasion, so he got me the kit I just mentioned. With one of our sons, an engineer who is also an AR owner, we put it together. It wasn’t tricky – you just need to pay attention. 

So You Want To Build An AR…

I am gearing this article to the first-time AR assembler, so we’ll keep things fairly basic. (There are tons of how-to videos and articles that cover specialized builds). After reading this article, you hopefully will be encouraged enough to order the parts and have at it yourself. So… how do you put one of these things together? 

Before that, here is a quick explanation for the new AR aficionado… your typical AR is comprised of two parts. There is the upper receiver, which houses the bolt carrier group (BCG), barrel, sights, everything north of the lower. The lower receiver holds the trigger, pistol grip, bolt and mag releases, safety, buttstock, and all the parts it takes to make those things work. For this opus, we’ll be looking at just the lower end of things. I will feature parts that are easily obtainable online or in many stores. 

What Will I Need? What About Legalities?

Here is a photo, below, of the basic parts that comprise the lower receiver. Bear in mind that the lower is the only part of the complete rifle that contains a serial number, so you will have to buy your lower, either stripped of complete, from an FFL holder. You can order uppers and any of the lower receiver component parts directly from a dealer. Of course, like everything else, there are exceptions to this but in general this is how it works.

Speaking of stripped and complete, what’s the difference? A stripped lower is just the steel (or aluminum) housing that all the parts fit in. A complete version has all the parts installed. This saves you time, but they are more expensive than a plain stripped lower, plus they don’t allow customization of parts. It’s your call. 

For this article, we will start with a stripped lower. Here’s one from the BCA site. Price: $64.99

Images from bearcreekarsenal.com unless otherwise noted

Notice that this is one of the bare-bones models, hence the word stripped. Moving on, we have the parts kit that is comprised of all the springs and other doohickies that fit in the above receiver…the kit’s price is $59.99.

Here is a breakdown of what’s in a typical parts kit, minus the grip…

(The photos above shows the trigger and related springs. We will use a complete trigger assembly, further below).

We’ll look at these parts in a little more detail a bit later. One part that sticks out and is very recognizable (even by beginners), however, is the pistol grip. There are many options where that is concerned, as there are for most all the parts you see here. We will stick with what’s here for the sake of simplicity.

We need a trigger. Here is a 3-pound, drop-in Velocity trigger from the BCA site. It will work very well for our build. Price: $149.95.

And, lastly, a buttstock is needed. Here’s one again from the BCA site, a Minimalist First Tactical stock that only weighs 6 ounces but is collapsible and has multiple sling points. Price: $49.99. (Not shown: the buffer and its spring. Those usually come with the stock).

So there you have it. To sum up, here’s what we need to build our lower receiver, and costs rounded off:

-stripped lower receiver     $ 65

-parts kit                                   $ 60

-trigger                                      $ 150

-buttstock                                 $ 50

Total:                                           $ 325

Bear in mind that complete lower receivers are fairly generic, by and large. That means that once you put one together, you can use it with pretty much whatever upper you stick on it. That’s the beauty of the AR platform… interchangeability is built into it. I’ve known guys who own just one lower receiver but many uppers. Spending money on a lower receiver build will save you money in the long run, and adding a custom trigger will only sweeten the deal - you’d get the same trigger pull for all your ARs. The exception would be the shooter who needs complete rifles ready to go out the door at a moment’s notice, but for many of us, one lower will suffice.

Tools Needed

Here is a list of tools that will help make your build easier. This list comes from thewarriorsolution.com.

●       Gun Vise

●       Real Avid Master Armorer’s Kit

●       Lower Receiver Vise Block

●       AR-15 Bench Block

●       Bolt Catch Roll Pin Set

●       Armor’s Wrench

●       Torque Wrench

●       Pliers

●       3/16 Hex Key

●       Masking Tape

●       Non-Marring Hammer (The Real Avid armor’s wrench is also the hammer.)

●       Roll Pin Tweezers

●       AR-TT (AR Takedown Tool)

●       Castle Nut Staking Punch (or other punch)

●       Precision Screwdriver Set (For the trigger guard set screw and other small screws)

●       Eye Protection

Optional tools and supplies include:

●       Wrench Pliers (For the bolt catch and trigger guard roll pins)

●       Gun Oil (Lube as you build, so you don’t have to do as much later.)

●       Razor Blade (For the pivot pin spring and detent)

●       Gun Grease (For the castle nut and buffer tube; also for barrel nuts)

●       Aluminum Black touch-up (For repairing scratches and blemishes)

●       Needle Nose Pliers (For holding roll pins)

●       Roll Pin Specific Punches (Makes driving roll pins suck less)

●       Deck Of Playing Cards (To use as shims to support the receiver if you don’t have a dedicated bench block or vise)

●       Inch Pound Torque Wrench (For the grip screw and trigger/hammer pin screws)

Those are the basic tools needed, with optionals thrown in. Many of the tools are pretty generic, except for AR-specific ones. You might want to borrow those tools from someone who does this a lot unless this is just the beginning of your relationship with ARs.

There are many great makers of gun tools out there, but I will single out one manufacturer of tools that has worked well for me… Real Avid. The items they’ve sent me over the years for review have been of top quality and have worked very well for the task at hand.

So, How Does It Go Together?

OK… time to build our AR. Let’s walk through the process. There is more than one way to do this, but let’s get the easy stuff out of the way first.

I will add at this point that a bench vise is essential. It’s hard to try to put a lower together on a kitchen table (ask me how I know). The best solution is to add a lower receiver vice block to use with your vise. That is a magazine-shaped block that goes into the mag well and then is clamped in the bench vise. It holds everything securely.

1. The Magazine Release, Catch, And Spring

First take the magazine catch, release, and spring out.

Insert the mag release into the left side of the receiver, in the large hole…

Next, put the mag spring (the fattest spring in the kit) into the right side of the receiver. The release button threads onto this.

Push and hold the magazine release like you were ejecting a magazine and make sure the textured side of the release button is facing out. Make sure the catch can spin freely. Using a punch, press the mag release button in to compress the spring and rotate the catch clockwise to tighten it. 

You’ll want to try different magazines to make sure the release is working properly. 

Please note that the mag release is different for pistol-caliber ARs. It releases and latches the mag the same way but is cantilevered in order to engage the pistol mags’ locking notch, which is located on the front edge of the magazine. It has to “reach out” more. Here’s my shot of the new BCA BC-45 .45ACP pistol’s mag release…

 This is a specialized case. The vast majority of AR builds will have the standard rifle-caliber release.

2. The Takedown/Pivot Pin

This is the forward pin that the gun hinges on when you unlock the rear latch. It can be a little fiddly to get in correctly, as it can launch spring-powered parts across the room if you’re not careful.

There are a few tools on the market that can help with this. One such tool that I found is the AR Takedown Tool (AR-TT). I wish I had known about this when I first started in the AR game. It is designed to make installing the takedown pin easier. Its spring/hammer action allows you to seat pins without having to whack them with your 20-ounce Estwing... 

This should simplify the process. For $40 including tax, you can reduce your headache potential. Or, you can pick up a ¼”x2” cleavis pin from your local hardware store and use that.

What you need to do is to put the spring and detent into the hole in the receiver without sending them across the room. 

Using the AR-TT helps. Come in from the right side and put the hole in the AR-TT over the hole in the receiver.

Now take the pivot pin spring and insert it through the AR-TT into the lower receiver. Then, install your detent with the pointy side facing out.

Once you get the spring and detent in place, unscrew the front part of the AR-TT tool, the small part facing away from the receiver. Push the small pin through the AR-TT just far enough to push the spring and detent into the receiver. Now, rotate the AR-TT 90 degrees, so the hole is vertical. The spring and detent will be set in the receiver by the AR-TT and won’t go flying.

To install the pivot pin, hold it so that the cutout part is facing the rear. Hold pressure on both the pin and the AR-TT at the same time and slide the pin over. Push them together, until you hear the pin click into place. The pivot pin is now set in place.

Here’s another option: Use something really thin like a razor blade to depress the spring. If you use a thin enough blade, you can slide the pivot pin over it and then pull the blade out. This is a decent way to go if you don’t want to buy a new tool.

Speaking of new tool, here’s an alternative to the AR-TT. It’s Real Avid’s pivot pin tool. For twelve bucks, you can prevent parts springing into space. Here is is…

3. The Bolt Catch

For this operation, a Real Avid Bench Block comes in handy. Again, this is something you could borrow unless you’re going to do a lot of these.

The bolt release goes on the left side, just above the mag release. If you don’t have a bench block, secure the lower gently in a padded vise or by other means.  

The bolt catch pin needs to be installed from the back. If you put it in from the front, you might damage it by bending it or breaking off its “ears” during installation. Notice the masking tape - that helps protect the receiver’s finish.

This shows a Real Avid roll pin punch starter. This lets you start the pin (it’s hollow and holds the pin). Once you get it started, switch to a regular punch or the Kinex pliers mentioned above.

4. The Trigger Guard

The back of the trigger guard is the fatter part that goes toward the grip. Most set screws come preloaded with a thread locker and will screw right into the receiver itself.

Please support the lobes of the trigger guard when you are seating this pin – if you break them, you have to go buy a new stripped lower. The above-mentioned bench block has a support built into it, or a deck of cards works – just position it under the trigger guard as you whack the pin into place. The card advantage is that you can shift them to exactly match the contour of your guard.

Remember the AR-TT? You can use it as your “whacker”. It’s self-centering, so you retract the middle post after placing it over the pin and let it go…

5. The Trigger

It’s pretty simple. It’s a drop-in part that’s fairly self-contained. Here’s an illustration with another brand of trigger:

Add the pins and secure them with their setscrews and you’re done. 

6. Safety & Pistol Grip

The safety goes in from the left side of the receiver. It should slide right in, but if it doesn’t, press down on the hammer to move things around and let the safety seat. 

Now, the grip. Make sure the selector spring shown is installed:

… and then slide the grip onto the receiver. There’s a hole right below the safety selector where the spring goes. Make sure it’s seated there:

Safety detent: make sure the pointed part goes towards the safety selector. The flat part goes down towards the grip. As you push the safety selector through the hole, make sure that it is in either the safe or fire positions. If you have it over to the other side, it’s not going to click into the detent. When you push your grip up, make sure that you aren’t pinching the spring – don’t bind it. Just slowly push the grip straight up onto the lower receiver.

Pull the bottom cap off the grip and install the set screw. 

7. Takedown Pin, Buffer Tube, & Butter Retainer

From the right side, place the takedown pin with the groove towards the back into the hole above the safety selector.

The takedown detent pin goes in next, in the hole in the back of the receiver. This is the tiny hole on the right.

Next, put the takedown spring into the back of the receiver:

Now, for the buffer retainer and its spring…

The retainer and spring go like this…

… and are placed in this hole, spring first:

8. Buffer Tube and Castle Nut

First, make sure the castle nut is completely on the tube. It doesn’t have to be tight at this stage but just on all the way. 

Now, start screwing in the receiver extension, which is the buffer tube, into the lower receiver. Don’t over-tighten it.

When the lip of the receiver extension touches the buffer retainer, take a punch and depress the spring. At the same time, screw in the receiver extension. 

The lip of the buffer tube is going to capture the buffer retainer. Like above, don’t over-tighten the receiver extension –  just get the lip of it over the buffer retainer.  

Now, compress the receiver end plate against the lower and screw down the castle nut. Be careful and don’t go all gorilla on it…

9. Tighten And Stake The Castle Nut

Here’s where you need a couple of specialized tools: an armorer’s wrench, and a torque wrench. 

You can get the armorer’s wrench from several sources… one comes in the Real Avid Armorer’s Kit, or you can order one separately. Put the lower in a vise, and then cinch the castle nut to around 40 ft./lbs. (38 - 42 is the spec). You might want to put some protective grease behind the nut to prevent corrosion. Now, we have to stake the nut in place… here’s how to do that.

Use a good center punch or a staking tool. Keep the lower in your vise and displace metal as shown above from the receiver plate to the castle nut. Do this in two places. You can take things apart if needed if you stake it.

10. Install Buttstock And Buffer Spring

To do this, we’ll use a generic Mag-Pul buttstock for illustration. You have to depress the lever in the middle of the stock in order to put the stock on the tube 

Squeeze the extension handle and pull down on the bars on the side with a pair of pliers. This will allow the stock to slide onto the tube.

11. The Last Step: Install The Buffer And Spring

Slide the spring into the tube. The buffer retainer should “click” when the spring and buffer are in place.

That’s it – you’re done!

Here is your new, complete lower receiver. Now, all you have to do is add an upper. Stay tuned!