My First AR-15 Lower Build

A newb's experiences assembling an AR-15 stripped lower.

Written by rtfjr86
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I bought my stripped lower receiver from Aero Precision in September 2017. Now, two and a half years and 3 kids later, I finally have assembled my lower receiver. This article is all about the experience and lessons learned by a newb assembling his first AR-15 lower. I have only ever owned handguns and this is my first time owning and assembling an AR-15. I could have just purchased a rifle off the shelf, but I wanted to learn the ins and outs of the rifle and assemble everything myself just as a personal challenge. This will also allow me to assemble the firearm as time and finances allow. If you want to skip ahead and check out the upper receiver build and the completed rife, click here: AR-15 Upper Receiver. With that said, let’s get into it.

Parts List

  • Aero Precision stripped lower receiver (gen 2) - $110
  • ALG Defence Advanced Combat Trigger - $69
  • CMMG lower parts kit - $40
  • Aero Precision AR15 Enhanced Carbine Buffer Kit - $47
  • Magpul MOE SL stock - $57
  • Magpul MOE+ grip - $24

Some of the Parts For My Lower Build

The Build

As a guide, I followed the assembly instructions posted over at Pew Pew Tactical. You can view that post here.

First I stalled the magazine catch. Very easy and straightforward. I did over tighten the catch by one turn which caused the catch to be pointing out too far of the magazine release button, but that was an easy fix. Next, I moved on to the trigger guard. I have the Aero Precision Billet Trigger Guard which installs with three screws as opposed to pins. Super simple.

Next, I moved on to the bolt catch. This was a bit tricky. I used a punch from the back side to hold the catch in place while I started the pin with a punch from the front. Once the pin was started, I switched to needle nose pliers to squeeze the pin in the rest of the way. The pliers were wrapped in an old t-shirt to minimize scuffing.  In hindsight, I should have installed the pin from the rear where there is a lot more room to work.

From there, I installed the trigger. The ALG Defence Advanced Combat Trigger came with the trigger and disconnector already assembled. So that was a fairy easy drop in. I was easily able to hammer the trigger pin in with a nylon hammer. The hammer was a little more difficult to install, but not by much. The hardest thing for me as a newb was making sure the hammer springs were properly seated inside of the receiver. Per the guide I was following, once the hammer pin was started, I gently dropped the hammer to relieve some pressure on the pin and used a punch to push it the rest of the way through.

I then went to install the safety selector - WITHOUT A GRIP! This being my first build, I had no idea the safety selector, detent, and spring were all held in place by the grip. I ordered a lower parts kit minus a grip mainly because the standard A2 grip looks awful and I wanted to FDE all the things. First lesson learned: RTFM all the way through and make sure you have all of the necessary parts (and tools) before you begin. Luckily, I was able to quickly run out to Sportsman’s Warehouse and pick up a Magpul MOE+. The install itself was a piece of cake.

Next I installed the buffer tube and rear takedown pin. I did not know that the rear takedown pin was held in place by the buffer tube endplate. Luckily, I actually had a buffer tube ready to install unlike my grip mistake. I first screwed in the buffer tube, then installed the rear takedown pin. A mistake I made was to not watch closely enough the rear takedown spring. I screwed the castle nut down until the endplate was firmly against the back of the receiver. Then I checked the rear takedown pin. It was loose. Dammit! I loosened up the endplate and sure enough the rear takedown spring was nowhere to be found. After digging around in the carpet for a few minutes I found it. Damaged. Having been run over by my desk chair. I bent it back into shape as best I could, stuck it back into the receiver, and tightened down the endplate much more carefully this time. I slid the Magpul SL stock on without any trouble.


Bent Rear Takedown Pin


Bent Rear Takedown Pin Being Installed


Now, the final part to install to complete my stripped lower build, the front pivot pin. Turns out the final part was the most difficult part without the right tools. I tried using my pocket knife blade to hold the detent and spring in place while pushing in the pivot pin. However, the knife blade was still too thick to allow me to push the pivot pin into place. After chasing the spring and pin twice across the room, I decided (as the guide suggested) to try one last time inside of a cardboard box so if the spring and detent went flying, I wouldn’t have to go chase them. Well, the box did its job when the pin and spring went flying for the third time. I was done at that point. Again, I was lucky Sportsman's Warehouse had an AR-15 pivot pin install tool in stock. I was able to install the pivot pin within a minute using the tool.


Pivot Pin Install Tool


And that is it. I completed my first AR-15 lower build from a stripped lower. Will I do it again? Probably not if I don’t have to. I will probably just purchase a complete lower in the future unless I want something super custom. I learned a lot about the functioning of an AR-15 and now have the confidence to do maintenance or make modifications as I see fit. On to a stripped upper! My wallet hurts already. 

Lessons Learned

  1. Read through all the instructions and parts lists before beginning.
  2. A pivot pin tool and punch kit are essential tools.
  3. Be patient. There are a lot of small parts under a lot of tension and you may have to make multiple attempts at an install.
  4. Have fun and learn something.

The Final Product

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