How to Build an AR-15 | Stripped Lower Receiver Assembly

Our how to build an AR-15’s guide will help you build your first AR-15 – whether it’s a billet or forged lower. No matter the brand – the basics of AR-15 lower receiver assembly remain the same.

Why build an AR-15 lower receiver

Building an AR-15 lower receiver and subsequent rifle can be very rewarding. The satisfaction from building a well running rifle is great. It can also save you money.

The lessons you learn along the way will help you when it’s time to replace key parts.

Where to buy an AR-15 stripped lower receiver

Your local gun shop will be more than helpful in finding you a stripped lower receiver. You can also find these on manufacturer’s websites and GunBroker.com. Local gun shops can also help you assemble your rifle or problem solve any issues you have along the way.

Need a recommendation? We list your best options here.

Tools you’ll need:

  • Needle nose pliers
  • Armorer’s wrench
  • Hex key set
  • Punches
  • Mag block
  • Vise

Parts you’ll need

  • Stripped lower receiver
  • Buffer tube kit
  • Lower parts kit
  • Stock

How to Build an AR-15

Before you start your AR Build

Unbox and inspect your parts – look for:

  • Blemishes
  • Missing pieces
  • Confirm the parts are for an AR-15, not an AR-10
  • Confirm your buffer tube and stock will fit (commercial vs mil-spec)

You may get to a point in your build where you need a detent spring or detent pin and cannot find it. I have yet to find a lower parts kit that didn’t have all the necessary parts. Usually, it’s my fault that something is missing or misplaced.

Find yourself a good gun dealer who stocks these small parts or may give you a spare from an old build or leftover parts. If not, you’ll buy the part for $0.40 and pay $3.00 in shipping costs.

Step 1. Install magazine catch assembly

This is by far the easiest place to start.

Find these parts from your lower parts kit. Take your lower receiver and hold it in your hand.

Insert the magazine catch into the left side of the lower receiver. It will only fit on one side.

On the right side, place the spring over the magazine catch rod.

Place the magazine catch button over the spring. The magazine catch button has a smooth and a textured side. The smooth or non-textured side connects with the magazine catch rod. Twist the button clockwise and it’ll start to screw onto the magazine catch.

To tighten beyond the button, we suggest the following method. Turn the lower receiver so the button is facing you work space. Place a punch on table with the punch inside the center of the lower receiver’s magazine release button.

This will push the magazine catch out of the receiver. Slowly start to rotate the catch clockwise. Do this cautiously so you don’t scratch the lower’s finish. Continue to turn until any further turns will scratch the surface.

Return to magazine catch to its space in the lower receiver. To test everything works properly, enter an unloaded magazine. Press the magazine catch button to make sure it releases the magazine. Another important test is to make sure the button cannot turn turn onto the face of the receiver, making it impossible to release the magazine. In it’s “resting state”, it should stay inside the recess of the lower receiver.

Step 2. Install trigger guard assembly (if necessary)

Some lower receivers have a built in trigger guard – this is commonly the case with forged lower receivers.

If you have a billet receiver, you’ll need to install a trigger guard. With this build, Aero Precision included a trigger guard. It came with screws and the lower was threaded to receive these screws.

ar-15-trigger-guard-install

If your receive doesn’t come with threads and screws, you’ll need to punch the trigger guard roll pin into the receiver. Be careful not to scratch the surface or break the receiver during this process.

Step 3. Install bolt catch

Next is bolt catch installation.

Aero Precision’s M4E1 has a threaded bolt catch pin that’s removable with a hex key. I wasn’t aware that this one didn’t have that feature. Luckily we had leftovers to use on this build.

You’ll install the bolt catch using a roll pin punch. Again, carefully punch this into the hole for the bolt catch roll pin.

You’ll carefully punch this in to get it started. Don’t let it get through the first space – you’ll need to put the bolt catch into that space.

Insert the bolt catch spring into the hole for the bolt catch release. On top of it, place the bolt catch buffer with the pin side down. The rounded “nipple” should face away from the lower.

Place the bolt catch over these pieces and in between the slots for the bolt catch.

Line up the bolt catch release so the roll pin can go through. Start punching the roll pin through. Once finished, test the catch – does it move freely? Does it spring back into position? If so, installation was successful.

Step 4. Install trigger assembly

The trigger assembly is usually the most difficult part for me. Pressing down the springs, maneuvering the parts – it’s tough for me. I installed it the first time without issues but to take photos, I needed to uninstall. The second attempt at installation was much more difficult.

Installing the trigger and hammer spring

Below we’ve shown pictures of how the springs will sit on the trigger and hammer. Put these on as shown below.

Trigger install

Grab the trigger and the bolt catch spring. Insert the spring into the recessed area at the rear of the trigger. You may notice one side of the spring is larger than the other. Only one side with comfortably fit inside the trigger. Insert that smaller end in there.

Next, grab the disconnector. There’s a small notch machined in the disconnector. The round hole in the side of the disconnect will line up (with some pressure on the spring) with the trigger’s.

Insert the trigger into the lower receivers trigger area with the curved side facing the front of the receiver. With a free hand, grab the trigger pin and insert on the side of the lower receiver. Sometimes, you can push it through to the other side without issues.

You may need to use a hammer to bush the pin all the way through.

Hammer install

The hammer install takes a bit of strength. Insert the hammer with the “claw end” facing the rear of the receiver. The legs of the hammer spring will go on the sides of the receiver.

The best way to do this is to put the legs of hammer spring onto the outside edge inside of the lower.

Press down so tension begins to fight you. Move the hammer to the rear, “dragging the spring” along the bottom edge of the receiver. Bend until the holes in the hammer align with the receiver’s. Push the hammer pins into the receiver, using a hammer to finish installation if necessary.

Step 5. Install pistol grip

Next you’ll install the pistol grip. It’s impressive to understand all the functions that depend on that pistol grip’s installation.

Step 5A. Install safety selector switch

Install the safety selector switch on the left side of the receiver.

Next, there’s a detent screw that goes into a channel in the safety selector switch. Turn the receiver upside down and install this detent, with the pointed end going into the receiver.

Step 5B. Install pistol grip

Grab your pistol grip. You’ll notice a small hole in the top portion of it. You’ll install a thin tightly coiled spring into the hole.

As you install the pistol grip into the receiver, the spring will go into the hole that you pushed the detent into. Be sure the spring doesn’t bend during installation.

Grab a hex key that fits the the pistol grip screw and fasten the pistol grip to the receiver.

Step 6. Install rear and front takedown pins and detents

The rear takedown pin works in conjunction with the buffer tube kit. The front takedown pin is the most difficult to install. I read about a trick though that makes it very easy though and hopefully you find it easy as well.

Front takedown pin installation

Put the takedown detent spring in the small hole at the fore end of the receiver. It will stick out beyond the receiver.

Take a pair of needle nose pliers and firmly grab the top of the detent. Put the detent at the top of the detent spring and firmly push down.

Grab a razor blade and put it on top of the detent. Push the detent down into the hole at the hole in the front of the receiver.

Once fully pushed down, grab your front takedown pin and insert it, channel side towards the rear of the receiver – and remove the razor blade. The spring will push the detent into the pin’s channel.

Rear takedown pin installation

Install the rear pin on the right of the receiver with the pin’s channel facing the rear of the receiver. At the back of the receiver is a small hole to the lower right of where the buffer tube screws in.

Add the detent. Next add the spring.

Step 7. Install buffer tube kit

Insert the buffer retainer spring into the rear of the lower receiver where the buffer tube screws in. Next, add the buffer retainer to the top of the spring.

Install the castle nut and end plate

We’re using an Aero Precision Enhanced Buffer Tube Kit. This buffer tube kit has a small area cut out of the lower portion of the buffer tube kit. Other buffer tube kits are cut straight. This one is made to fit very snugly.

First screw on the castle nut. Next, add the end plate. The raised portion goes into the receiver -it covers the cutouts on the receiver.

Begin to screw the buffer tube into the lower receiver. Eventually you’ll hit the buffer retainer. You’ll need to press this down to continue screwing the buffer tube into the lower.

Be careful – the buffer retainer will be under spring pressure – it will shoot out

I lost the spring while taking photos of this build.

Continue to screw the buffer tube into the lower until the buffer retainer is at the rear of the hole cut in the buffer tube. Each time you rotate the buffer tube, you’ll need to press down the buffer retainer and make sure it doesn’t shoot out.

Press the endplate to the end of the lower receiver. Screw the castle nut tightly against the receiver. Tighten with an armorer’s wrench.

Step 8. Insert buffer spring and buffer into buffer tube

Place the buffer spring into the buffer tube.

Install the buffer into the buffer tube housing

It will take some additional pushing to get the buffer totally inside the buffer tube.

Step 9. Install buttstock

Using the manufacturer’s instructions, install the buttstock. We plan to use a fixed stock on this build. Maybe you’re building a pistol braced or collapsable stock AR15.

Wanting an even more compact home defense solution?

Step 10. Sear, trigger, trigger reset and takedown pins tests

Complete the following tests before using complete lower receiver.

Sear test

Set the lower to safe as shown in this photo. Make sure the trigger assembly is reset and the hammer is down. Press the trigger – does the hammer release? If not, test was successful.

Trigger test

Set the lower to fire (the vertical position, pointing up). Press the trigger. Does it release the hammer? If so, the test was successful.

Trigger reset test

Reset the trigger by pushing the hammer back down. Make sure the lower is off safe. Squeeze the trigger but do not release it.

Next, reset the hammer back to the down position. Slowly release the trigger. You should feel a click – this is the trigger resetting.

Takedown pin test

On the left side of the receiver, press in the front and rear takedown pin. They should move almost the entire way out of the lower. The detents we installed will keep them in the lower receiver.

Once extended, push back inside. If these move in and out reasonably well (they can be tough to remove), then the pins have passed the test.

What next?

You’ll want to start building or purchase an upper receiver. We’ll write a guide on building an upper receiver in the future. The upper receiver assembly can be just as easy as the lower receiver assembly.

Did we miss something? Need a more in depth guide? Check out PewPewTactical’s guide.

Purchase components

You might want to look at purchasing the other components in the meantime. Here’s a quick list:

  • bolt carrier group (don’t worry about bolt carrier assembly – bolt carrier groups arrive assembled)
  • gas tube and gas tube roll pin as your gas system
  • gas block
  • upper receiver (stripped upper or assembled upper with forward assist and ejection port cover assembly installed)
  • there are many others to list.

Rifle build planning

Keep in mind how you want this rifle to look and shoot.

Sights

Do you want to use a scope, red dot, or iron sights?

Barrels

Are you going to build a rifle length AR-15? Carbine? Short barreled rifle? What material barrel will you choose? Which brand?

Gas system lengths

Depending on your barrel length, you’ll need to to decide which barrel or gas system length fits your plans.

Caliber

We always recommend to customers they keep in mind which ammunition is available most easily and at a lower cost.

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