How to Build an AR-15 Lower Receiver: Ultimate Visual Guide

There’s nothing like assembling your own AR-15 lower receiver for personal satisfaction and customization.

I’ll guide you through everything with minimal tools and tons of tips I’ve learned the hard way!

So get ready for A LOT of pictures that will make building your lower a breeze.  Plus a bonus of the upgrades I immediately did to my lower to make it perfect for 3-Gun competitions.

But first…a disclaimer that this is for educational use only.  If you don’t have the right tools or experience, please have this done by a professional or buy a completed rifle.

Parts and Tools

Parts

All AR-15 Lower Receiver Parts
All AR-15 Lower Receiver Parts

I went with a California featureless build so there’s the funky looking grip.  But otherwise everything is pretty standard.

I haven’t found much difference between lower parts kits and buffer tube assembly’s as long as there’s decent reviews and not a no-name company:

Lower Kit, ar15.com
Lower Kit, ar15.com

Tools

  • Punch Set ($15): I prefer brass so you don’t mar up your lower receiver and it’s good for lots of other gun stuff too
  • Long Arm Hex Key Set ($12): You’ll be using hex keys or Allen wrenches in a lot of installations.  Go with the long arm versions for extra reach.
  • Hammer ($10): I like the rubber/plastic ends instead of standard steel since there’s very few times when you actually need that much force…also if you use it directly on gun parts, you will not mar the finish
  • C-Clamp ($10): Not super necessary but useful to install the trigger guard, 4-inch should be good
  • AR-15 Combo Tool ($20): Good for installation of the castle nut, but more useful for upper receiver builds
  • Utility Knife ($7): Needed for the pivot pin installation

Alright, let’s get onto the actual build!

Magazine Catch Assembly

Magazine Catch Assembly Parts
Magazine Catch Assembly Parts

You can start your AR-15 lower receiver build many different ways but I like knocking out some of the easier ones first for quick wins.  Find these three parts along with your lower receiver.  The magazine spring is the largest width spring in your kit.

Magazine Release Spring
Magazine Release Spring

Place the magazine release spring into the hole shown above.

Magazine Release Button
Magazine Release Button

Place the magazine release button on top of it with the serrated side up.

Magazine Catch
Magazine Catch

Press in the magazine release button and flip the receiver over to start threading in the magazine catch.  It might be a little tricky the first time you do it since you have to maintain decent pressure on the button to make it flush against the receiver while matching up the threads.  Righty-tighty!

Make sure the catch doesn’t scrape against the raised portions of the receiver.

Screwed in Magazine Release
Screwed in Magazine Release

Once you have it threaded slightly you can release the pressure and rest your fingers for a little.  Then get ready to do it again!  Press down the magazine release even further into the receiver past the flush line and tighten the magazine catch a couple more times.

Gap Between Magazine Catch and Receiver
Gap Between Magazine Catch and Receiver

Check once in a while.  Here you can see that there’s a gap between the mag catch and the receiver when you’re pressing the mag release button flush.  You’ll need to tighten more or else the mag catch could fail to go back into the receiver during a reload.

Correct Distance for Magazine Catch
Correct Distance for Magazine Catch

This is a good distance where the magazine catch will not leave the receiver unless somehow you manage to really smash in the magazine release button.  You can test with some of your mags to make sure it catches and releases.

Congrats!  You’ve installed the first section of your AR-15 lower receiver.

Trigger Guard Assembly

Trigger Guard Assembly Parts
Trigger Guard Assembly Parts

Find the trigger guard and the longer roll pin.

Depress Pin of Trigger Guard
Depress Pin of Trigger Guard

Depress the pin of the trigger guard which will be closer to the magazine well.  Don’t worry, the two sides are differently sized so you can’t mess it up.  Then just slip it in.

One Side of Trigger Guard In
One Side of Trigger Guard In

Sometimes I find that the other side has a hard time dropping in so I lightly press down or even use the plastic hammer (lightly) if it’s slightly more stuck.

Roll Pin for Trigger Guard
Roll Pin for Trigger Guard

Make sure everything is lined up for the roll pin.

Start Roll Pin with Hammer
Start Roll Pin with Hammer

You can either start it with the hammer, with a punch, or just with your fingers.  I like using the hammer to get started then using the C-Clamp below.  It’s much easier than trying to hold down the receiver while hammering.

Note: If you decide to go with just hammering…make sure you are supporting the dog ears or else they can pop off pretty easily even on a good quality lower.

Finish with C Clamp
Finish with C Clamp

I did end up marring my finish a little so I would heavily suggest using some plastic (I’ve heard a little piece of milk carton plastic between both sides of the clamp will do wonders).

Installed Trigger Guard
Installed Trigger Guard

The installed trigger guard.  You can see a little damage to the finish but I know I’ll be dealing a whole lot more damage to the receiver at the range and during competitions.

Bolt Catch Assembly

Bolt Catch Assembly Parts
Bolt Catch Assembly Parts

Find these parts!  The bolt catch spring is even throughout while the other remaining smaller spring is fat on one end for the disconnector.

Bolt Catch Spring and Plunger
Bolt Catch Spring and Plunger

Place the spring on the plunger and drop into the hole above the magazine latch.  There better not be comments on me needing a manicure…

Bolt Catch
Bolt Catch

Orient the bolt catch like above and drop it down.

Bolt Catch Roll Pin
Bolt Catch Roll Pin

You will need to depress the bolt catch for everything to line up.  I like using a hammer and punch to get it started.

Squeezing the Bolt Catch Pin
Squeezing the Bolt Catch Pin

Apparently there’s a special tool to clamp the roll pin but I decided to make my own.  However, it wasn’t protective enough so I would suggest wrapping your pliers with tape first then some cloth.  Or you can just continue using the punch all the way through.

Installed Bolt Catch Pin
Installed Bolt Catch Pin

The finished bolt catch with just a little bit of marring.

That wasn’t so bad right?  The trigger coming up is a little tricky but don’t worry, I got you!

Trigger Assembly

AR-15 Trigger Assembly Parts
AR-15 Trigger Assembly Parts

Pick out these parts from what’s left.

Correct Trigger Spring Orientation
Correct Trigger Spring Orientation

Attach the springs according to what you see above.

Disconnector Spring In Fat Side Down
Disconnector Spring In Fat Side Down

Place the fat side down into the trigger so that the spring stays put.

Disconnector Placement
Disconnector Placement

Place the disconnector onto the spring.

Trigger Hole
Trigger Hole

Press down on the disconnector to see the trigger hole.  You’ll be placing this assembly into the receiver and then putting a pin through it.

Placement of Trigger
Placement of Trigger

Place the assembly so that the trigger goes through the oval bottom of the receiver.

Trigger Hole Through Receiver
Trigger Hole Through Receiver

Match up everything so that you can see the clear hole for the pin.

Trigger Pin Almost Through Receiver
Trigger Pin Almost Through Receiver

I sometimes find the trigger pin has some trouble once it’s almost through.  Sometimes I resort to using a brass punch after matching it up.

Installed First Trigger Pin
Installed First Trigger Pin

Here you see the fully installed first trigger pin.

Orientation of Trigger Hammer
Orientation of Trigger Hammer

Double-check the placement of the hammer and spring.  Make sure the springs lay on top of the first trigger pin.  They’ll be decent pressure to align the second trigger pin hole.

Trigger Hammer Pin Halfway
Trigger Hammer Pin Halfway

Once you get the trigger pin halfway through, I like cocking the hammer down so that it eases the pressure a little bit.

Trigger Hammer Pin with Punch
Trigger Hammer Pin with Punch

It turned out extra stiff so I used the brass punch.

Installed Trigger Hammer Cocked
Installed Trigger Hammer Cocked

Here’s the installed trigger assembly.

Release Trigger with Finger
Release Trigger with Finger

Resist the urge to dry fire!  You’ll release the hammer which has nothing to hit except the lower receiver.  Do this enough and it will start to damage it.  I hold the hammer down with my other hand so that it slowly releases instead of slamming into the receiver wall.

And there you have it…the trigger assembly!

Safety, Takedown Pin, Grip, & Receiver Extension

Still with me?  We’re in the home stretch now.

Safety, Takedown Pins, Grip, Receiver Extension
Safety, Takedown Pins, Grip, Receiver Extension

These should be the remaining parts in your kit.  Not the different sizes of springs relative to each other.  The two skinnier longer ones are identical for the takedown and pivot pins.

Grip Holes
Grip Holes

Let’s start with the grip.  The top one is a normal pistol grip while the bottom is the CA legal Hammerhead grip.  Both have holes for a screw and washer.  You might just need a longer hex key for the pistol grip to reach all the way in.

Cock Hammer Back
Cock Hammer Back

But first, let’s cock the hammer back so it opens up the safety selector hole.

Insert Safety Selector
Insert Safety Selector

Insert the safety selector.

Set Safety to Fire
Set Safety to Fire

Set the position to “Fire”

Safety Detent
Safety Detent

Find the safety detent (pointed side into the receiver) and put it in the hole on the opposite side of the safety indicator.

Safety Detent Inserted
Safety Detent Inserted

A much better view of the silver safety detent installed.

Spring for Safety Detent
Spring for Safety Detent

Find the safety detent spring.  It’s the thinner one that is slightly shorter than the other identical thin springs.

Insert Spring into Grip
Insert Spring into Grip

Insert it into the hole at the top of whatever grip you’re going to use.

Spreading Apart Hammerhead Grip
Spreading Apart Hammerhead Grip

Sometimes the grip doesn’t fit on easily (nor do you want it to be loose).  I find that using a big flathead makes it easier to slide onto the lower receiver.

Matching Up Spring with Safety Selector Hole
Matching Up Spring with Safety Selector Hole

Match up the spring with the safety selector hole and double-check that the safety selector is still in the “Fire” position on the opposite side.

Screwing in Grip
Screwing in Grip

Screw in the grip with the enclosed hex nut and washer.

Shorter Rear Takedown Pin
Shorter Rear Takedown Pin

Look for the shorter pin (rear takedown pin) which will have identical spring and detent to the longer pin (pivot pin).

Install Rear Takedown Pin and Detent
Install Rear Takedown Pin and Detent

Install the rear takedown pin from the left to right (shown fuzzily, sorry!) and also place the detent in the back of the receiver.  It helps to orient the takedown pin so its “channel” will accept the detent.

Insert Detent Spring
Insert Detent Spring

Insert the detent spring.

Buffer Retainer and Spring
Buffer Retainer and Spring

Now find the buffer retainer and spring (thickest one left) and place it in the hole above.

Placement of End Plate
Placement of End Plate

Also take a look at your end place where one side has protrusions which will fit nicely with the back of the receiver.

Castle Nut First
Castle Nut First

But first, screw on the castle nut in the position above.  With the bigger square cuts toward the back of the lower receiver.

End Plate on Buffer Tube
End Plate on Buffer Tube

Then put on the end plate in the correct orientation.  I’ve definitely done this in the wrong order, so please learn from my fails!

Screwing in Buffer Tube
Screwing in Buffer Tube

Start screwing in the buffer tube.  The end plate will move with each turn and start messing with the spring.  Try your best to not let it come out.

Almost Done Screwing in Buffer Tube
Almost Done Screwing in Buffer Tube

When you’re almost done screwing in the buffer tube, take a break to look at the other side.

Buffer Tube Almost to Buffer Retainer
Buffer Tube Almost to Buffer Retainer

You want to stop right before the buffer tube reaches the buffer retainer.

Buffer Retainer Active
Buffer Retainer Active

Perfect fit with just a little bit more of a turn.

Match Up End Place
Match Up End Place

Now you need to match up the end plate with the receiver and also make sure that when the plate is tightened that it will hold the spring.

Screw in Castle Nut
Screw in Castle Nut

Start screwing in the castle nut with your hands first.

Tightening Castle Nut
Tightening Castle Nut

If you have an AR-15 armorer’s tool, it will have a section to tighten the square cuts.  I’d heavily recommend that since it’s only a few bucks.  But otherwise you can also just lightly punch it tight.

Staked End Plate, ar15.com
Staked End Plate, ar15.com

You can also “stake” your castle nut where you use a center punch to smash in the end plate into the smaller cuts of the castle nut.  I would recommend testing out your lower a lot beforehand since it’s annoying to undo, but really keeps it together.  Also make sure your end plate allows staking.

Front Pivot Pin
Front Pivot Pin

Now find the last pin, screw, and detent that will be for the front pivot pin.

Detent Spring In
Detent Spring In

Place the detent spring in first.

Detent In
Detent In

Followed by the detent.

Pliers and Razor to Hold Detent
Pliers and Razor to Hold Detent

Time to whip out the pliers and utility knife to hold the pin down flush against the receiver since you’ll be sliding the pivot pin over.  I’d recommend doing this in a sideways cardboard box or something above you to catch the potential flying detent and spring if you mess up.  Also, you might want some eye protection.

Insert Front Pivot Pin
Insert Front Pivot Pin

Insert the front pivot pin over the knife blade with the channel facing where the detent will spring out.  This will install the pivot pin.

Completed Front Pivot Pin
Completed Front Pivot Pin

Ta-dah!

Stock, Buffer Spring, and Buffer
Stock, Buffer Spring, and Buffer

Now find your stock and buffer and buffer spring.  Yes, it’s super ugly but I got it since it was the cheapest “color.”

Buffer Spring In
Buffer Spring In

Insert the buffer spring in.

Buffer End Out
Buffer End Out

The buffer end will orient like the above.

Slide on Stock
Slide on Stock

Slide on the stock until it doesn’t move easily.

Pull Tab to Engage
Pull Tab to Engage

Pull the adjustment tab out and continue moving the stock inwards.  You can now use the adjustment tab to select your preferred distance.

And YOU’RE DONE!

Unless you’re in CA and want to pin your stock, then check out my How to Pin a Collapsible Stock Guide.

But if you’re like me…you want to make your AR-15 lower receiver the best it can be.  Here’s the upgrades I added immediately and that I detail in my Ultimate 3-Gun Rifle Build article (coming soon).

Ultimate 3 Gun Rifle Build
Ultimate 3 Gun Rifle Build

AR-15 Lower Receiver Upgrades

AR-15 Lower Upgrades
AR-15 Lower Upgrades
  • JP Silent Captured Spring ($140): One of the best upgrades to make your AR-15 feel like a pellet gun!  It takes away the *sproing* of the traditional buffer tube and drastically reduces recoil.  If you’re going with a lighter BCG, I’d recommend getting it with the additional springs so you can truly tune your gun.  Otherwise, the stock spring works great.  See my in-depth review here (coming soon).  Also available on JP’s official site.  And how to Install & Tune the JP Silent Captured Spring.
Slide in JP Silent Captured Spring
JP Silent Captured Spring
Screw in Ambi Safety
Screw in Ambi Safety
  • Magpul BAD Lever ($30): On every single one of my lower receivers.  Makes manipulating the bolt super easy from your right hand.  Install it in less than 5 minutes.
Assembled BAD Lever
Assembled BAD Lever
  • Hiperfire Trigger ($275 & $225): Swapping out the trigger is probably the best upgrade you can do to your AR-15.  This one is a new single-stage trigger that has no perceptible creep, little reset, adjustable weights (I run mine at the lightest 2.5lb), and actually increases the hammer strike power.  One of the weaknesses of other triggers is that while you can get light pulls, those sometimes can lead to light primer strikes too.  See my in-depth review.  The installation was also much easier than I expected given the amount of parts.
Hiperfire Eclipse Trigger
Hiperfire Eclipse Trigger
Hipershoe Installed
Hiperfire Installed
Hiperfire Eclipse 2.5 lb Trigger Pull
Hiperfire Eclipse 2.5 lb Trigger Pull

Conclusion

There you have it…the complete visual guide to building you own AR-15 lower receiver!  Pair it up with our How to Build an AR-15 Upper Receiver and also check out the guides below for more how-to’s and recommendations.

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31 Comments on "How to Build an AR-15 Lower Receiver: Ultimate Visual Guide"

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Terry Howard
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Just finished my build this was a great guide. Thanks

Jeff Thalin
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Would you please talk about lubrication?
How do you feel about pre-lubing the parts for protection before assembly?
What parts do you feel require lubrication?

Steve
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First ever lower build and it was done in under thirty minutes! Thanks for a great guide!

mike turbo
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just finished my first custom build, and thanks to you and your site it went flawlessly.

much respect and appreciation.
michael

Vincent
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Great instructions. Used this from start to finish and didn’t scratch the gun at all. Was a lot of fun and gives you the knowledge to find problems later on. Thank you.

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