Aero_Precision_Lower_Receiver_AR15The AR15 is one of the most ubiquitous firearms of the 21st century. Part of this popularity stems from its LEGO like modularity. Users can customize a rifle, or even build one from the ground up. While there are a lot of choices you can make when it comes to how to configure your AR15, the basis of each build is the lower receiver. Here I’ll cover how to install the Lower Parts Kit, or LPK, and cover some tips on how to easily get everything in place.

In this build guide, there’s only a handful of elements you actually need to buy. The AR15 lower receiver is one, but all those little bits and pieces come packed together in a Lower Parts Kit. I purchased one from Palmetto State Armory that exchanged the usual A2 grip and straight trigger guard for a Magpul MOE Grip and MOE Trigger Guard. The other item purchased here was a Phase 5 Tactical Pistol Buffer. You’ll need a stock kit to complete the lower; they are available from basic M4 stocks all the way to advanced modular systems.


It doesn’t really matter where you begin when putting together an AR lower. There are a couple of steps that are wholly independent of each other.

1) Front Pivot Pin (Takedown Pin). This can be installed at any time.

2) Magazine Catch and Bolt Stop (Bolt Catch Lever). These can be installed independent of other assemblies, but the Magazine Catch should be installed before you pin in the Bolt Stop.

3) Fire Control Group and Selector (Safety). You can put these in before or after the previously mentioned parts. However, you will need to install the Trigger, then the Hammer, then the Fire Selector.

3b) Pistol Grip. This should be installed after the fire selector.

4) Buffer Tube, Receiver End Plate, Rear Takedown Pin. These can be installed at any time, but you can’t really put in the rear takedown pin until you have a stock to hold everything in place.

I usually don’t use a lot of tools for this job. You can use pins and punches and whatnot, but, saving for those spots mentioned below, I usually just poke stuff with my fingers (and this has never gotten me in trouble o.0).

Hammer, Trigger, Selector (Safety)

I’ll start with the fire control group. Take the trigger and the disconnector spring. The disconnector spring fits into a notch on the trigger. Fit the disconnector on top and make sure it travels smoothly. The spring shouldn’t bind in the hole. Fit the trigger spring over the trigger posts. This can take some prying, but it should snap into place.


Drop the trigger into the lower and press through the trigger pin. In most cases, the trigger pin and hammer pin are identical. Grab one of the two you have and push it into place.


Next is the hammer. The hammer spring fits over the posts and rests against the back of the hammer. The tails of the hammer spring rest over the trigger pin. I balance the hammer with the spring in place above the fire control opening and press the back of the hammer to push it into place.  Since there’s a bit of force here, getting it lined up can take a little bit of work. Once you have the holes in line, push in the hammer pin.


Now you can test to make sure the hammer locks. Pull it back and make sure it resets like it should. However, you should not drop the hammer without catching it. Letting the hammer impact the section in the middle of the receiver can damage the receiver. You should catch the hammer by hand when testing, and only dry fire when the completed rifle is assembled.

Aero_Precision_Lower_Receiver_Assembly_Guide (2)

AR15_Selector_Lever_Safety_Installation_GuideWith the hammer cocked, slide the fire selector into place. Turn the receiver over; the fire selector detent will drop into its place in the vertical hole above the pistol grip mount. The pistol grip has a hole for the fire selector detent spring. The spring will press up into the corresponding channel on the receiver, so it’s best to slide this into place straight to prevent pinching the spring. Once it’s all the way seated, tighten the screw inside the grip. The grip in this kit installed with a standard flat head screwdriver.


Magazine Release

AR15_Magazine_Release_AssemblyAt this point I jumped over to the magazine release. Put the magazine release in place and slide the magazine release spring onto the shaft. Turn your magazine catch button once or twice, then line it up with the hole in the receiver. In order to tighten the mag catch fully, press the button as far as you can into the receiver. On the other side, spin the magazine catch clockwise until the end is flush with the face of the button. This is much easier when the bolt stop is not in place, since the mag catch spins through the spot where the bolt catch lever sits.


One of the more difficult parts is the front takedown pin, or pivot pin. Some people will use a tool here, but I’ve found it’s straightforward to just use the included components. Drop the long spring into the channel. I balance the pivot pin detent in front of the spring. There’s a small groove that will prevent the detent from rolling. Using the tip of the pivot pin, press the detent into the hole. This way, as soon as the detent is recessed enough, you can slide the pivot pin into place.



Bolt Stop (Bolt Catch)

AR15_LPK_Bolt_Stop_Detent_PaddleThe next two parts are the first place I use tools, and they’re a little more difficult than the previous items, especially if you’re trying to avoid scratching the finish on the lower. Take the bolt stop detent spring and drop it into the hole inside the groove for the bolt stop; drop the detent in above the spring. The bolt stop, or bolt release, or bolt catch, will fit over these. Before I put the bolt stop in place, I like to start the roll-pin into the hole so it’s already aligned. Some people here will use a polymer tip mallet or polycarb punch to hammer the pin into place.

There is another way, which is what I did. I protected the back of the protrusion the bolt stop fits in with a folded rag, and used a set of channel locks to start the roll pin. Once the roll pin is just into one side of the channel, I fit the bolt stop into place. When I resume pressing the roll pin into place, I go slowly and check to make sure the bolt stop is aligned and the pin is locking the bolt stop in place correctly. Don’t go too fast or you risk jamming the pin into the side of the bolt stop. I completed the task using channel locks and tapped the pin in a little farther using a punch.



Trigger Guard

AR15_Trigger_Guard_MOE_Trigger_Guard_InstallationThe trigger guard is where I usually take the most care. The front of the trigger guard is simple. In the case of the MOE, it’s fixed with an allen set screw. With USGI trigger guards, they have a little pop-up button that clicks into place. The rear is where you should be careful. Here a roll pin fastens the back of the trigger guard in the receiver. The tang that sticks down here is thin. If you are going to hammer the pin or use a punch, you MUST make sure you support the tang here. Many a disappointed AR15 enthusiast have broken the tang off their receivers by doing this step improperly. Usually I will support the section right underneath the roll pin hole with a section of wood. For this build, I used the rag and channel locks again. Whether you use a hammer and punch or the channel locks, this is where you should learn to listen to your equipment. Don’t jam something when a little part of you says, “Maybe you should straighten that a little before you force it.”

Buffer Tube (Receiver Extension), Receiver End Plate, Rear Takedown Pin

AR15_Phase_5_Tactical_Pistol_Buffer_KitThe last part is the buffer tube, or receiver extension, with the receiver end plate and rear takedown pin. Since this build is designed as an AR pistol, I’ve selected the Phase 5 Tactical Pistol Buffer Kit. This buffer kit includes the buffer and carbine buffer spring. Check your kit, but most will include these parts, along with the castle nut and receiver extension tube, or buffer tube. I liked the fact that the Phase 5 buffer includes the foam cheek pad.

The sling mount I’m using is the SLAP, or Sling Loop Ambidextrous Plate. These are available from, and are reviewed elsewhere on the site.

First, thread the castle nut onto the buffer tube; screw it on almost as far as the threads go. This will be far enough to let the receiver end plate clear the detent spring. Slip your chosen receiver end end plate onto the buffer tube after the castle nut. Now you can thread the buffer tube into the receiver. Note here that if you get excited and slip the receiver end plate over the buffer, then thread it into the receiver, you’ll have to back and take everything apart to put the castle nut on. Just get it in the right order the first time, it’s faster.


Thread the buffer tube into the receiver until the front edge of the tube approaches the hole for the buffer retention detent. Drop the buffer detent spring into the hole, then press the buffer retention detent in on top of it. While pressing this into the hole, thread the buffer tube in until it traps the detent in place. All buffer tubes should have a groove that will align the tube properly using the notch on the receiver end plate. Some have a design that will index off of the detent.


AR15_Rear_Takedown_Pin_Detent_SpringNow we can add the rear takedown pin. The pin slides in from the side of the receiver. Drop the rear takedown pin detent in the hole in the rear of the receiver, followed by the detent spring. The Aero Precision lower used here has a unique feature in that it has a threaded hole. A set screw can be used to contain the detent  and spring. On most lowers, the receiver end plate will contain the spring. Simply slide the still-loose receiver end plate onto the end of the receiver, and finally tighten the castle nut. This will keep everything locked into place.


You will need a castle nut tool to properly tighten the stock. The manual specifies torquing this nut to 40 ft. lb. of torque. However, many shooters will forgo the use of a torque wrench. It is possible to tighten the stock and use a combination of blue loc-tite and proper staking to keep the castle nut in place. Whether you go by the manual, or whether you use German Spec (Gutentaight) and staking, make sure it won’t come loose. If it does, you’ll end up in a bad spot, likely at the worst possible time.


Your lower receiver is now complete. Since my build uses a pistol buffer, it is legal to pair with my 11.5 inch upper receiver group. Whatever your application, your lower is done! Grab your upper receiver, bolt carrier group, and get it out there!

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