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

Table of Contents

There are lots of good reasons to build your own AR-15 lower receiver. Some do it for the savings —piecing together a lower from individual parts can save you anywhere from a few bucks to a few hundred, depending on components you choose. Others do it to have complete control over the final product. If there’s a particular combination of stock, trigger, grip, and small parts that you want, building is often the only way to get it without retrofitting an already built lower.

Luckily, building your own lower receiver is not a difficult task; if you know how to use a few common hand tools, you can build an AR-15 lower.

Despite this, it can also be easy to mess up and damage or mar your receiver if you’re not familiar with the process. For that reason, we’ve put together this handy guide to help even the greenest AR builders piece their lowers together without a problem. We’ll go through the whole thing, step by step.

ar 15 build exploded parts

Step 0: Tools and Parts

Before you can get down to building your lower, you’ll need to do some prep work. For this task, that means gathering the tools and parts required and setting up a workspace.


  • Allen Keys—Most lower receivers will require at least one Allen key, but depending on your model, more may be needed.
  • Punches—As with Allen keys, standard AR-15 lower receivers will require the use of several punches, although some avoid this by replacing roll pins with small screws. You can get by with standard punches but roll pin punches will make the process easier.
  • Staking Punch—This specialized punch is used to secure the castle nut in place. It’s technically optional, but very highly recommended if you plan to use your rifle for anything more serious than plinking. Alternatively, some castle nuts utilize a ratcheting mechanism and do not require staking, such as the PWS version.
  • Hammer—For driving punches and seating stubborn pins. A specialized hammer with non-marring striking surfaces is recommended, but optional.
  • Armorer’s/Castle Nut Wrench—Required to install the castle nut. Most Armorer’s wrenches have a castle nut wrench built-in and offer excellent value, but if you’re on a budget, you can pick up a standalone castle nut wrench very affordably.
  • Tweezers/Needle Nose Pliers—These can be substituted for forceps or any other similar tool. They’re not required, but they can be handy for holding pins in place while installing them, especially if you have large fingers.
  • Vice & Receiver Block—Also optional, but incredibly convenient. Using a vice and a lower receiver block to hold your lower in place frees up both your hands to work with the parts, which is a huge advantage while installing those tiny springs and pins. .
  • Bench Block—Optional, but heavily recommended. A bench block will provide a solid, non-marring surface for your receiver to rest on while driving in pins. Some are even designed to accommodate the uneven angles of AR-15s, saving you a lot of headache.
  • Magnetic Parts Tray—The last of our non-essential conveniences. A magnetic tray makes storing the smallest parts, like pins, detents, and springs, simple and frustration-free while you work.

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  • Lower Receiver
  • Fire Control Group—This should consist of a trigger, hammer, disconnector, and the associated springs and pins. Alternatively, you can buy a drop-in trigger, but you will usually need to buy the trigger pins separately.
  • Magazine Catch
  • Magazine Button
  • Magazine Button Spring
  • Bolt Catch
  • Bolt Catch Spring
  • Bolt Catch Plunger
  • Bolt Catch Roll Pin
  • Safety Selector
  • Safety Selector Spring
  • Safety Selector Detent
  • Trigger Guard—Many AR-15 lowers now come with an integrated trigger guard, in which case you can skip this part.
  • Trigger Guard Roll Pin
  • Grip
  • Grip Screw & Washer—Some AR-15 grips do not require a washer or have one molded into the grip itself, but most will require a separate washer.
  • Buffer Tube—You may also see this part called a receiver extension. They are available in commercial and Mil-Spec versions, but we recommend Mil-Spec, as there are a greater variety of stocks available for that diameter
  • Buffer—Make sure to match the weight of your buffer to the gas system length of your intended upper receiver. Usually, shorter gas systems will require heavier buffers. If you don’t yet know what upper you will be using, an H buffer is a safe bet for a wide variety of barrel and gas system lengths.
  • Recoil Spring
  • Buffer Retainer Pin
  • Buffer Retainer Pin Spring
  • End Plate
  • Castle Nut
  • Stock—Make sure your stock is matched to the diameter of your buffer tube.
  • Front and Rear Takedown Pins
  • Takedown Pin Springs and Detents

Or, of course, you can just buy a lower parts kit that bundles all of these parts together.

Setting Up Your Workspace

Now that you have all your parts and tools assembled, it’s time to configure a place to work on your lower receiver. If you opted for a lower receiver block, clamp it into your vice and slot your receiver onto it.

vise block work bench

If not, you’ll want to start by finding a sturdy, flat surface to work on. A kitchen table can be used, but there will be a bit of hammering in this process, so a protective covering like a gun mat is recommended to avoid damaging the surface. A workbench, of course, would be ideal.

Organize your parts so you can find them easily and reach them without moving—there’s nothing more frustrating than holding an obstinate spring and detent in place and being unable to reach the takedown pin meant to secure them. Your tools should be arrayed similarly.

Lastly, make sure your workspace is well-lit, clear of clutter, and if possible, uncarpeted. It’s easy to send a spring or detent flying by accident, particularly if this is your first time assembling an AR-15 lower. Should that happen, the parts will be much easier to locate on tile or hardwood than tangled up in the fibers of a carpet.

With that finished, it’s time to start building.

Step 1: Installing the Magazine Catch

Starting by placing the magazine catch into its slot on the left side of the receiver. Then, from the other side of the receiver, insert the magazine catch spring over the threaded end of the magazine catch.

ar15 lower magazine button install

Next, take the magazine catch button and use it to compress the spring. Twist it on until it catches the threads, then continue turning only until it aligns with its slot in the receiver. You’ll want to hold the magazine catch in place from the left-hand side. Be careful not to turn the magazine button too far, or you’ll scratch the receiver.

Now partially installed, the magazine assembly should hold itself together. At this point, push the magazine button in as far as you can and hold it there, then rotate the magazine catch to thread it into the button.

Continue threading until the end of the magazine catch sits just below flush with the surface of the magazine button and the catch is aligned with its slot. Release the magazine button and let the catch settle into place.

ar pcc assembled lower magwell

In most cases, this will complete the installation, but it’s important to double-check the magazine catch engagement to be sure. The magazine catch should now sit flush or slightly below flush with the left side of the receiver. When the magazine button is pressed flush with the right side, the hook of the magazine catch should entirely exit the interior of the magazine well but should not clear the exterior wall.

Step 2: Installing the Trigger Guard

Depending on the exact design of your trigger guard, installation may vary. At the front of your trigger guard, you will typically have either an Allen screw or a spring-loaded plunger. For the former, use an Allen key to back the screw out and remove it. For the latter, you can simply depress the plunger.

Insert the trigger guard into its proper location in the receiver, aligning both sets of holes, then release the plunger or reinstall the Allen screw.

For this next step, you will usually want to remove your lower from your receiver block, if you’re using one. If you have a bench block, set the receiver in the appropriate slot to support the trigger guard area.

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If not, you’ll need to find something to prop up the underside of the receiver near the grip to prevent it from wobbling during the next step. Failure to do so can result in damage to your receiver.

Now that we’re at the point of installing our first roll pin, it’s time to talk about tape. It’s not uncommon for a roll pin to remove some of the finish from around the hole it’s inserted into during the installation process. While this isn’t really a functional issue, it can be a cosmetic one.

To avoid that, masking or painters take can be used to protect the receiver. Simple place a small piece of tape over the hole, just large enough to cover it completely, then drive the roll pin through the tape. The pin will puncture the tape to enter, forming the tape over the edges of the hole and protecting the finish. After driving the pin through, remove the small piece of tape that is punched out from the other side.

With the receiver properly supported and the trigger guard aligned with the holes in the receiver, drive in the trigger guard roll pin.

Step 3: Installing the Bolt Catch Assembly

To install the bolt catch, start by inserting the bolt catch plunger into the bolt catch spring. Then, insert both into the appropriate hole in the lower receiver.

Now take the bolt catch itself and insert it into the slot to check the fitment. You should be able to press it in enough to align the hole in the bolt catch with the holes in the lower receiver. If you are not able to align the holes, your lower receiver is likely out of spec, or your bolt catch spring is too long.

bolt catch roll pin install ar 15 lower

Assuming everything fits, you can remove the bolt catch and start a roll pin. Use a roll pin starter punch or “Walt’s tool” if you have one available.

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With the roll pin started, replace the bolt catch in its slot and align it with the holes in the receiver. Then, drive the roll pin into place. A properly seated bolt catch pin should be flush with the surface of the lower receiver on either side of the catch.

Step 4: Installing the Trigger Assembly

The procedure for installing the trigger assembly can vary considerably depending on the make and model. These instructions will assume you are using a standard Mil-Spec trigger. If you are using a drop-in trigger for your build or a unique design such as a Hiperfire, refer to the manufacturer’s instructions for installation.

primary arms ar 15 lower drop in trigger

Start by assembling the trigger and disconnector. The trigger spring should fit onto the trigger shoe as shown below, with the U-shaped portion resting under the front of the trigger, the loops secured in place around the round protrusions on the sides, and the legs facing forwards. Install the disconnector with the hook facing forwards and the disconnector spring fitted into both the round slot in the trigger and the square one in the disconnector.

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Now, holding the parts together so that the holes in the trigger and disconnector align, put the whole thing into the lower receiver, so that the trigger shoe sticks out through the slot into the trigger guard area. You’ll usually need to do a bit of adjusting to make sure everything lines up properly, but once it does and you can see through the holes in the receiver, trigger, and disconnector, go ahead and push a trigger pin through the hole to secure it all in place, starting from the left-hand side.

If you’re having trouble keeping everything aligned, you can insert a punch of similar diameter from the other side to hold the parts in place while you drive in the pin.

In most cases, the trigger pin can be pushed in with finger strength, but for tight fits or strong springs, a light tap with a hammer may be necessary. Be absolutely sure that everything is properly aligned before applying force with a hammer, and tap gently. If the pin is not going through, do not force it. Just take out the assembly and start again.

Next, install the hammer spring onto the hammer. The flat section of the hammer should be facing forward, and the hook or point will face rearward. Install the spring so that the U-shaped portion presses against the back of the hammer, the legs also face down or back, and it is secured on the side protrusions of the hammer in the same manner as the spring on the trigger.

The legs of the spring need to rest on top of the trigger, so you’ll have to compress the spring a bit as you install the hammer. This can make getting the hammer aligned with the pin hole in the receiver a bit tricky, so it’s generally recommended to press the hammer down and let it catch on the disconnector to take some of the pressure off. As with the trigger, align the holes and drive in the pin. Again, do not force the pin as something is likely misaligned. Trying to force a pin in can potentially cause damage.

Do not dry fire at this stage, as dropping the hammer on your unprotected receiver can damage it. If you must check the trigger function before proceeding, wrap your hand around the magazine well so that your index finger sits between the hammer and the bolt catch. This will allow the hammer to fall on your finger rather than the receiver, preventing damage. But it may hurt a bit.

Step 5: Safety and Grip

With the hammer cocked back, insert the safety selector from the left-hand side of the receiver. Then, holding the safety in place, flip the receiver over and drop the detent into the small hole near the grip area, pointy end first.

ar 15 pistol grip and safety spring lower install

Now we’ll need to get our grip assembled. Install the screw and washer into the slot in the grip and hold it in place with an Allen wrench or screwdriver. Place the safety detent spring into the appropriate hole in the grip as well.

Now, press the grip into place on the lower receiver, just far enough to allow the threads of the grip screw to engage. Give the grip screw a few turns to secure it, but don’t install it fully.

With the grip partially installed, check the alignment of the safety detent spring. Adjust it if necessary until it is straight.

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Finish screwing in the grip screw, watching the safety detent spring to ensure it doesn’t bind or catch. Once the grip is firmly in place, test the safety.

Grip the magwell as previously described to protect the receiver, then cock back the hammer if necessary and set the safety selector to the “safe” position. Try pulling the trigger. You should not be able to release the hammer.

Now turn the safety to the fire position and pull the trigger again. This time, the hammer should fall. Repeat this test as necessary until you are confident in the function of your safety.

Step 6: Install the Takedown Pins and Buffer Tube

We’ll start with the pivot pin (the front pin) since it’s the simplest. To install it, you’ll need a pivot pin tool, such as the one available from Real Avid.

Remove the detent plunger and large plastic pin from the tool, then install it on the lower so that the flat part of the tool lays against the front of the magazine well. Then, reinsert the large polymer pin from the side of the receiver that does not house the detent.

Load the detent spring through the hole in the pivot pin tool and into your AR-15 lower, followed by the detent and plunger. Rotate the pivot pin tool 90 degrees. Insert your pivot pin into the tool and remove the detent, pressing the large plastic pin out of the way with the pivot pin. This will allow the detent to lock against the pivot pin rather than the tool.

Remove the large plastic pin, then the pivot pin tool. Test the function of the pivot pin by pushing it into its closed position, then back to open. It should snap into each position, but should not be able to be pulled free of the firearm.

With the installation of the pivot pin behind you, the rest of the assembly will be a breeze. Assemble the buffer tube next by threading on the castle nut with the side with the smaller notches facing the receiver and the larger notches facing back towards the stock. Thread the nut on as far back as it will go. Then, place the end plate onto the buffer tube with the oval indentation facing the lower receiver. This indentation will match up with an oval cutout on the receiver to keep it indexed.

ar 15 castle nut buffer plate

Screw the buffer tube into the receiver until you almost reach the hole at the bottom of the threads. Place your buffer detent spring inside of your buffer detent, then place both in the hole. Hold it down and screw the buffer tube in until the lip of the tube covers the flat section of the detent, but not the point, and the buffer tube is properly oriented to the receiver.

Now, twist the buffer tube slightly to move the end plate out of the way of the takedown pin detent spring hold on the right of the rear of the receiver. Install the takedown pin through its holes, drop a detent in from the rear, and then slide the last detent spring in after it.

Hold the detent spring down and twist the buffer tube back into place. Press the endplate firmly against the receiver to compress the takedown pin detent spring into its hole, being careful not to pinch the spring. Screw the castle nut on until it is hand-tight to hole the end plate in place.

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Next, use your castle nut wrench to snug the nut up tightly. It doesn’t need to be extremely tight, like a barrel nut, but you want a decent amount of torque on the threads to help hold the nut in place. It is generally recommended to torque the castle nut to 38-42 ft/lbs.

If you want to stake your castle nut (which we heavily recommend with standard nuts), take your staking punch and do so now. Place the tip of the punch in the center of the endplate next to one of the small indents in the castle nut.

Give it a good hard strike from the hammer. You’ll get the best results if you do this in one swing, but if the initial indent isn’t as deep as you’d like, you can try again. The Mil-Spec standard is for the castle nut to be staked in two places, but most users and many commercial manufacturers find one to be sufficient.

Step 7: Install the Buffer and Recoil Spring

Install your buffer into your recoil spring by pressing it into one end of the spring until it clicks into place. The spring is symmetrical, so it doesn’t matter which end you choose.

gunsmith buffer

With the buffer fully connected to the recoil spring, slide the spring into the buffer tube. You may need to depress the buffer detent to get it started and to allow the buffer to pass.

Final Step: Install the Stock

The stock installation procedure will vary depending on the model you chose, but most stocks can be installed by simply pulling down the adjustment lever and pushing it onto the buffer tube.

magpul prs stock ar 15 lower build

With the stock in place, your AR-15 lower receiver is complete. All that is left now is to pair it with your upper receiver you just assembled and repeat the function checks to ensure everything works as it should, then hit the range

Of course, assembling the rifle is rarely the end of a build; if you find yourself wanting to add accessories or upgrade your rifle down the road, we have a guide for that as well.