AR-15 Fundamentals: Building the AR15 Stripped Lower Receiver

lower receiver parts kit

For some, building an AR15 lower is a scary thought, so below we have built a visual guide on building your first lower. The build process is easier than you would imagine, and once you build your first, you will look to build more in the future!

DISCLAIMER: If you are unsure of any build steps, or have no experience with building and maintaining an AR15, seek professional assistance. Tactical Supplies 4 Less bears no responsibility for any damage or incorrect assembly of your firearm. The following guide is for visual reference only. If you lack the proper tools and experience, please have your rifle assembled by a professional armorer.

lower receiver parts kit

Parts List:

  1. Stripped AR-15 Lower Receiver
  2. Lower Parts Kit (for our build we used the CMMG Gun Builders Kit, this does not include the trigger group, trigger guard or pistol grip)
    • Buffer Retainer Pin
    • Buffer Retainer Spring
    • Safety Selector
    • Bolt Catch Spring
    • Detent Spring
    • Bolt Catch PlungerBolt Catch
    • Disconnecter
    • Disconnecter Spring
    • Magazine Catch Spring
    • Bolt Catch Roll Pin
    • Magazine Release Button
    • Magazine Catch
    • Pivot Pin
    • Selector Spring
    • Safety Selector Detent
    • Rear Take Down Pin
    • Takedown Pin Detent
  3. Pistol Grip
  4. Trigger Guard (if not attached to pistol grip or built in to lower receiver)
  5. Trigger Group (Drop In or 3 piece trigger kit)
  6. Safety Selector (aftermarket ambi selector)
  7. Buffer Tube – Adjustable 6 or 7 position (if ordering Milspec buffer tube, you need Milspec stock to match. If you order Commercial buffer tube, then Commerical stock to match for sizing.
  8. Buffer Spring (carbine or rifle depending on your stock selection)
  9. Buffer (either Carbine or H weight buffer as a starting point if using adjustable stock. )
  10. Castle Nut (carbine adjustable stocks only, A2 rifle stocks do not use a castle nut)
  11. Carbine Stock Latch Plate (with sling mounts or not for adjustable stocks.  A2 rifle stocks do not use the latch plate)
  12. Carbine Stock (or A2 stock if using for your build with A2 buffer/buffer tube/spring)

Tools Needed:

  1. Small Punch
  2. Light Hammer (brass/ non marring preferred)
  3. Pliers
  4. Needlenose Pliers
  5. Tape to protect lower from scratches (painters, masking, duct, guerilla)
  6. Small Block to support trigger guard tangs
  7. Flat razor blade
  8. Castle nut spanner wrench (AR-15 wrench or if you don’t care a punch and hammer will install the castle nut)
  9. Screw driver long enough for pistol grip screw
  10. Misc allen keys / torx for alternative parts if they do not include installation tools.

The Build:

Step 1- Magazine Catch Assembly: To start, I prefer to install the magazine catch assembly first. Take the magazine catch, and drop it in through the left side of the lower. magcatchinstall

We then flip to the right side, place our magazine catch spring in to the magazine release button hole followed by the magazine release button.  Press the button inward so the catch mechanism is moved outward away from the lower receiver. (see picture bottom right below) Turn the mechanism clockwise to screw into the mag button to start threading it.  After two turns, you will need to use a punch, dowel or pen to depress the mag release button deeper into the receiver so you can continue to tighten the magazine catch.   Continue to tighten the magazine catch until it is just shy of being flush with the magazine button on the other side.  Usually 2 turns from being flush with the outside of the button will find the magic tightness.  Using an AR-15 magazine, we can function check our mag release button works before moving on to step 2. magcatchbuttoninstall1

Step 2- Bolt Catch Assembly: After the magazine catch, I prefer to install the bolt catch assembly. By installing this after the magazine catch, we are able to turn the magazine catch easier on the left side than if we waited for the bolt catch to be in the way. The first thing I do to prevent scratches when installing the roll pin is to place tape (masking, painters or duct) on the left side of the receiver on either side of the bolt catch area. This will keep your punch/hammer from scratching the lower. If you prefer to use pliers to install the roll pin instead of a punch, I will also wrap the pliers head in tape as well.  Placing the roll pin in the right side by hand first will all you to not worry about having it fall out while tapping it with the punch.
boltcatchinstall

To help ease the install, tap the roll pin in until flush with the right tang (you can see the roll pin just barely peaking through on the right side tang). Next the bolt catch spring will drop in the hole, followed by the bolt catch plunger, with the bigger head side facing out.
boltcatchinstall2

The last step is the bolt catch itself. For our lower, we are using the POF-USA Enhanced Bolt Catch. You will drop this in on top of the spring/plunger, press down from the side to line up the bolt catch roll pin hole with the lower receiver and either tap the roll pin in with a punch or press the roll pin in with pliers. Both variations shown below. Function check the bolt catch to verify there is not binding on the pivoting movement.
boltcatchinstall3

Step 3- Front Pivot Pin: The next part we will install is our front pivot spring. This is a simple install, however, if you are not careful you can send the small detent and spring flying across the room! The pivot pin detent spring will be dropped in first. (The front and rear pivot detent and spring are identical parts. The pivot pin itself is different from the front and rear. The front is easily distinguished by having a flat side notch in the pivot pin head and is longer in length. This allows the pin to press flush and recessed in the spring/detent against the lower.) After the spring, the pivot pin detent will be dropped in. To ease the install of the pivot pin, we will use a flat razor blade to press the pin down and hold the spring compressed while we insert our pivot pin. With the flat side of the pivot pin head facing down towards the razor, we can easily install the pin into the first tang.  Once the pin is partially in, remove the razor by pulling it through the side, releasing the spring/detent into the pivot pin.
frontpivotpininstall

You can now push the pivot pin all the way in to the closed position and check for function.
frontpivotpininstall2

Step 4- Trigger Group: In our guide, we are installing a POF-USA drop in trigger group. The 7 parts in the POF trigger kit are: trigger group,  Left and Right sideplates, Hammer Pin (tongue slots), Trigger Pin (threaded) and 2 screws. To install our trigger, drop the trigger group in to the lower receiver and line up the front and rear holes.  On the POF triggers, you may have to apply downward pressure on the trigger group to compress the orange bumpers to line up the holes. With the holes lined up, install the Hammer Pin in the front slot (you may need to tap in lightly with a hammer). After the Hammer Pin, install the threaded Trigger Pin in the rear lower hole.  Next we will attach our side plates, the right side plate has a flat cut across the top for those ambi lowers which have a bolt catch release on the right side.  The side plate will drop in to the tongue and groove of the Hammer Pin and then turn the side plate until the threaded hole lines up to insert your screw.  Give the screw a couple turns and then install the left side plate. triggerinstall1

Using both sets of torx keys supplied with your trigger kit, torque both screws down to 8 in/lbs.  Once tight, place your thumb over the hammer and check for function of your trigger releasing properly.  DO NOT LET YOUR HAMMER SLAM INTO THE BOLT CATCH OR RECEIVER.  This can mar or dent either part.triggerinstall2

Step 5- Safety Selector & Pistol Grip:  After the trigger is installed we can install our safety selector. Our parts kit comes with a single side safety selector, we chose to install a POF-USA ambi selector. First install the selector from the left side of the receiver.  It should slide through with a little bit of wiggle required. The selector will sit flush on the right side of our receiver when properly install.  To install our right side lever, line up the grooves and verify the selectors are both facing the same direction before installing the set screw with a 7/32 allen key. (I do not recommend using thread locker at this time until we fully function check our safety later on in the build.)  safetyselectorinstall

Now that our selector is installed, we need to install the selector spring and detent.  These parts require our pistol grip to install at the same time to hold them in place.  (Our Daniel Defense pistol grip has a trigger guard molded in.  Now is the time to install your trigger guard on the lower if it is a separate piece.  When tapping the roll pin in to the rear trigger guard tang, closest to the pistol grip, make sure the tangs have support behind them so they do not bend. The front trigger guard hole will be held by a small hex screw to ease replacing this part if necessary in the future.)

For our grip install, we need our safety selector spring, safety selector detent (this one is larger than your pivot pins and only installs 1 direction, pointed side first), pistol grip, pistol grip screw and lock washer if included and lastly our front trigger guard hex screw.

Begin by flipping our lower upside down and turn our safety selector to ‘Safe’ position. Drop the safety selector detent in, pointed side first into the left smaller hole.  Next, I recommend putting your pistol grip screw and washer on your screwdriver and running it through your grip to hold it in place.  Place the safety selector spring in the small hole on the pistol grip, line up the grip with the lower and begin to push the grip on to the lower. By partially install the pistol grip we are able to start threading our pistol grip screw in to the lower receiver. As we tighten this screw on, we can push the grip on further to the lower. Make sure the safety selector spring is properly feeding in to the small hole on the lower receiver and is not binding.  Tighten the pistol grip screw till snug and verify your safety selector functions and clicks in to ‘Safe’ and ‘Fire’.  Also check the hammer is not released on trigger pull when in ‘Safe’ and the hammer is released when in ‘Fire’. Make note, the selector cannot be rotated to ‘Safe’ unless hammer is cocked. Afterward, we can fully tighten the pistol grip screw and install our forward trigger guard hex screw. safetyselectorinstall2

Step 6- Buffer Tube and Rear Takedown Pin: One of our last steps to finishing our lower will be the buffer tube and rear takedown pin.  The parts you will need are:  rear takedown pin, takedown pin spring, takedown pin detent, buffer tube, buffer retainer, buffer retainer spring,  end plate and castle nut.buffertubeinstall1

Our first step is to thread the castle nut with cut outs facing to the rear on to our buffer tube  almost as far as it will go.  Then slide on your end plate with the bump facing toward the front. (this can either be a flat end plate, have a sling mount or QD plate) We then will thread the buffer tube on to our lower receiver a couple of turns.  At this point, we will drop our buffer retainer spring in to the rear most hole followed by the buffer retainer over top of the spring with the small tip facing upward.  Using needle nose pliers, carefully depress the buffer retainer and screw the buffer tube in to the lower receiver until the tube holds the buffer retainer in place and the tip is not contacting the buffer tube.  In our example the POF-USA Anti-Tilt buffer tube has a buffer detent lock which prevents the tube from rotating once installed.  The last picture below shows this detent lock feature. The buffer tube only needs to be screwed in enough to hold the buffer retainer in place.  If screwed in to far, the buffer will not allow the Upper Receiver to close properly. If the buffer does not protrude past the Lower Receiver section then it will not contact the Upper Receiver. I’ve always screwed mine in until flush or almost flush. buffertubeinstall2

The second step is the installation is our rear takedown pin.  You will need the rear takedown pin, takedown pin spring and takedown pin detent.  First install your takedown pin in to the lower.  We will then drop in the takedown detent followed by the takedown spring.  Using either needlenose pliers or your fingers, press the spring underneath the end plate and push the end plate flush to the lower receiver to hold the spring in place.  Tighten your castle nut against the lower receiver to keep the end plate locked down and check your rear takedown pin for proper function.  Lastly, using your castle nut wrench, torque your castle nut to 35-40 ft-lbs.buffertubeinstall3

Step 7- Buffer, Buffer Spring and Stock:  The final step in finishing our lower receiver. Insert the buffer in to either end of the buffer spring and then feed the spring in to the buffer tube. The hammer must be cocked back to allow the buffer to pass by and insert the buff past the detent to capture the buffer in the tube.  Lastly, each stock has a slightly different way to install, follow the manufacturers directions on which way the catch is operated for installation. 

Our completed lower receiver, ready to be attached to an upper receiver and then hit the firing range.  I hope this visual guide on building an AR-15 lower receiver is helpful for you. Now get to building!completedlower