lower assembly

AR-15 Lower Assembly [Hands-On Guide]

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Building your own AR-15 lower is the most complex part of an AR-15, but it really isn’t too bad.

With some basic tools, a couple of hours (for your first lower, maybe 30 minutes for your 3rd+), and this guide — you’ll quickly be able to assemble your AR-15 lower and get it ready for an upper!

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Required Tools

I use slightly more than the bare minimum tools because a couple of small upgrades makes everything a lot easier.

For the lower parts kit, there really isn’t much difference between all of the options out there. Some will come with enhanced bolt catches or trigger guards or won’t come with a trigger or grip. What fits you best depends on what trigger, what lower, and what grip you want to use. 

Mix and match so that you have the combination that you want.

I normally choose a better trigger than the stock mil-spec version included in an LPK and I normally use a different grip.

Aero Precision’s AR-15 Lower Parts Kit without a grip, trigger guard, or trigger is perfect for me.

For this build, I combined that LPK with a LaRue MBT-2s, a Magpul Trigger Guard, and a Mil-Spec A2 Grip I had in my spare parts box.

Personally, I strongly recommend:

Lastly, I used the cheapest Castle Nut Wrench ever made. I can’t find this one for sale anymore, but there are dozens that are similar. Whatever the cheapest one is more than enough.

tools to assemble a lower
Everything I use to assemble a lower.

Magazine Catch Assembly

First, find your mag catch parts — you’ll need the button, spring, and arm.

Magazine release button, spring, and arm
Magazine release button, spring, and arm.

Slide the arm into the magazine catch cutout on the left side of the lower and pass the threaded part of the arm through the hole to the other side.

left side of the lower
lower left side

Set the spring over the threaded part of the arm, the orientation of the spring doesn’t matter in this case.

spring in lower

Place the button on top and press down firmly. Once seated on the threaded arm, twist the button one full rotation to get the threads started and keep the spring captive.

press down on the magazine release

Turn your lower to the side and press down firmly on the magazine release button, exposing the arm from the lower.

ar 15 lower assembly

See the light? You need to screw the button down until the release arm no longer completely clears the lower.

If you can see daylight between the arm and the lower, that’s bad.

I like to press very firmly on the button and then turn the release arm like a clock arm to screw it down.

ar-15 assembly
magazine release is fully screwed
This is how it should look when the magazine release is fully screwed down and the release button is being pressed firmly.

Once seated, you’re finished with this part! Easy, right?

Trigger Guard Assembly

Not all lower come without a trigger guard. Don’t worry about this part if you already have one set up or forged into the lower itself.

Magpul Trigger Guard
Magpul Trigger Guard

I used a Magpul trigger guard and this one is a little different than most others. 

The rear is a normal roll pin. You should have two roll pins in your LPK, the trigger guard uses the larger of the two.

Upfront, Magpul uses a set screw while most others use a detent.

The detents are normally installed already and just require being slipped into place.

For the Magpul, align the trigger guard inside the housing and simply screw in the detent.

aero lower

The rear part uses a rolling pin. 

Brace the trigger guard housing on your bench or Smart Block.

Align the pin and the housing with the trigger guard and give it a good wack with the Accu-hammer (or whatever small hammer you’re using).

Make sure to support the ears of the housing so they don’t break.

building lower assembly
building ar 15 lower assembly

Once flush, it’s done!

Bolt Catch Assembly

First, grab the parts (you saw that coming, right?).

You’ll need the catch, spring, weird plunger, and smaller of the two roll pins you started with.

Bolt Catch Assembly
Bolt catch parts

Set the plunger on top of the spring and place them into the hole inside the bolt catch area. You don’t really need tweezers for that, I’m just using them to show how they fit together.

Bolt catch parts
putting bolt catch parts

Place the bolt catch on top of the spring and plunger. 

Place the roll pin into the starter punch (the punch that has a cup at the end instead of a flat surface). This lets the roll pin drop down into the punch a short way and makes installing it a lot easier.

If you don’t have a starter punch… use a normal one.

assembly tools
lower

Set the punch with the roll pin against the lower so that the cutout part of the punch is facing the lower.

Align it with the roll pinhole of the bolt catch and give it a good wack with a hammer.

This is a three-handed step but if you have a decent-sized hand, you can hold the punch, the catch, and the lower with one hand and the hammer with your other.

AR 15 Lower Assembly

If you have small hands… Well, good luck.

Once the roll pin is mostly in, you can switch to a normal punch and finish driving it home.

lower ar15

Once flush, you’re done!

lower assembly ar15

Trigger Assembly

You’ll need the hammer, the trigger, hammer, and trigger springs, the two trigger pins, the disconnector, and the disconnector spring.

Trigger Assembly

Assemble the springs on the trigger and hammer-like in the picture. 

Take your trigger and place the disconnector spring into the hole so the fat side of the spring is in the bottom.

disconnector spring
Note the slight taper to the disconnector spring.
disconnector on top of the trigger

Next, set the disconnector on top of the trigger and then the trigger down into the lower.

Trigger Assembly
Line up the hole in the trigger with the lower

Line up the hole in the trigger with the lower of the two trigger pinholes. Press one of the pins into the hole until it is flush. 

Most trigger pins don’t need a hammer, but some do. If yours does — start off with gentle hits. 

Sometimes you think you need a hammer when really things are just misaligned.

If light hammer taps don’t drive the pin through, make sure things are lined upright.

lower ar15 image
ar15 lower
lower assembly

Once flush, you can move on!

The hammer springs are a little more annoying than the trigger was. The long parts of the trigger spring need to fold back to give spring tension.

Line them up as in the picture and force the hammer forward until it is lined with the front trigger pinhole.

force the hammer forward until it is lined with the front trigger pin hole
hammer spring folded back
Look closely in the middle and you can see the hammer spring folded back.

Once lined up, set the pin and push it through. Again, it should go in with just your hands but you might need a hammer.

Start off gently. It’s very common for this pin to get misaligned on the far side.

misaligned trigger
This is what a misaligned trigger pin looks like. Just wiggle it around until it is lined up and push it home.
fixing misaligned trigger

Done!

Safety & Grip 

This is everything left over at this point

Safety & Grip Assembly

Something to ID first though are the takedown detents and the safety detent.

The two takedown detents are copies of each other, but the safety one is larger and has a pointed nose with a flat base.

takedown and safety detents
The top two are the takedown detents, the bottom one is the safety detent.

Knowing the difference between these helps in a moment.

You will also have four springs. Two are identical, these are for the takedown pins, and one is a little shorter and a little thicker with tight rings, this is for safety. The fourth is about the same length but has much looser rings and is used for the buffer spring retention plunger.

I like to start with safety and grip.

Set your safety in the hole with the safety selector set to “fire”. It will take some wiggling and pressure to get it through and flush with the other side.

DO NOT USE A HAMMER. While it will take some wiggle and jiggle, it should never need a hammer. If you can’t get it through, it’s because it isn’t lined up.

If you need to, grab a punch and use it to press down on the trigger bar under the safety.

Aero Precision LLC Lower

Flip the lower over and place the safety detent with the point towards the safety.

aero
grip

Take your grip and set the detent spring into the small hole at the top of the grip.

grip assembly
spring in grip

Carefully hold the lower so that the trigger is facing the ceiling and connect the grip and lower. Slide them so the spring is pressed against the safety detent.

connect the grip and lower

Holding them together, install the grip screw.

Be careful with the grip screw and don’t cross-thread it. The screw is steel and the lower is aluminum and you can really mess up the hole if you let it cross-thread and force it.

AR grip assembly
Screw until tight and done!
AR-15 lower and grip

Takedown Pins & Buffer Tube Assembly

Start with your butter tube. Slide the castle nut on with the large cutouts facing the rear of the buffer tube.

Screw down the castle nut all of or at least most of the way back.

buffer tube

Slide the retention plate over that with the tab set into the grove of the buffer tube and the raised tab facing the front of the buffer tube.

Buffer Tube

Grab your lower and set the smaller of the two takedown pins into the rear takedown pinhole. Go in from right to left and face the grove towards the rear.

parts

See that hole in the picture above? The small one on the right-hand side of the back of the lower?

Place one of the detents in that hole followed by one of the springs.

detent should fit into the grove of the takedown pin

Careful not to dump these out. The detent should fit into the grove of the takedown pin. Wiggle the takedown pin so that these are lined up.

Twist in your buffer tube into the lower. Careful not to knock the spring out.

Keep going until the buffer tube is just about to cover the buffer retention hole.

Twist in your buffer tube into the lower
buffer tube into the lower
This is where to stop screwing it in. See how there is a tab that sticks out from the buffer tube? Stopping with this tab just short of the hole is where you want to stop.

Install the retention spring with the retention pin on top into the hole. Twist the buffer tube slightly more so that the tab of the buffer tube slides over the retention pin and traps it in place.

tube into the lower
buffer tube hole

Slide the retention plate down until flush with your lower. Hand tightens the castle nut by pressing it down until it is secure against the plate.

Use a castle nut wrench to give it a good twist.

Slide the retention plate down until flush with your lower
ar 15 lower assembly

If you want to stake your castle nut, you can. Personally, I normally don’t. A little bit of Loctite on the threads of the castle nut is a great way to secure it in place without having to stake it.

ar 15 build
One last step to go!

The front takedown pin is a huge pain in the ass. Don’t be shocked if you send a few detents flying.

If you’re doing more than one lower in your lifetime, you might want to get a Real Avid Takedown Pin tool.

Otherwise, a dull kitchen knife or an Exacto knife will help.

On the front of the lower, drop the last takedown spring into the hole.

Using some tweezers or needle-nose pliers, press the detent onto the spring and hold it in place using a knife.

ar 15 lower assembly - drop the last takedown spring
ar 15 lower assembly procedure

And, yes, it is far harder than it sounds.

Once you trap it, slide the takedown pin over the knife. Remove the knife and press the takedown pin the rest of the way.

a step in ar 15 lower assembly

Last but not least, just drop the buffer spring into the tube, set the weight on top, and cram it all down in there until it is past the retention pin.

drop the buffer spring into the tube
Install your stock

Install your stock according to the manufacturer. Most of them just slide on the rear of the tube. You may need to raise a lever or pin to get it started.

done ar lower assembly

Once installed — that’s it! You’re done.

Wrapping Up

See, now that wasn’t so bad — right?

It gets a lot easier once you do it a few times, but overall this is a pretty smooth AR-15 Lower Assembly process.

Let us know if you have questions in the comments!

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