Let’s Build A Mil-Spec AR Lower (With Just a Few Tools)


If you’re thinking about building an AR lower with a mil-spec parts kit and just a few tools, you have come to the right place. Today, we’re going to slam one together and break it down step by step.

Let’s not do the whole, “In this article, we’ll discuss” jive; here’s what you need:

  • A lower
  • A mil-spec lower parts kit (with FCG and Grip).
  • A castle nut wrench.
  • 2 (or 3) brass punches and a couple of steel ones. Sizes: 1/8″ and 1/16″ will get you by, add a 3/32″ for extra precision.
  • A brass hammer and a steel one.
  • Either a large Phillips or flathead screwdriver or an Allen wrench/ hex driver on a ratchet. This is all for whatever your grip needs to be attached with. Mine required a hex driver, but the parts kit comes with regular old screws.
  • A pocket (flathead) screwdriver.
  • A pair of tweezers would be handy.
  • A zip tie (we’ll get to that later).

The Stuff

Here’s what we have going on:

So, you might actually be looking at all of this stuff like this at first:

But you want to start separating that and sort things out. Specifically, organize all of your springs, detents, and assorted small parts. Once these are out and organized, don’t sneeze.. and make sure you have them in a container.

Of course, I didn’t do that; we just went for it, and every five minutes or so, I had to inspect the floor to find what I had dropped. You can avoid all of that or not—life is all about choices.

Anyway, these things:

If this is your first time staring at all this stuff, that’s okay; we’ll break it down. These parts have nuances, but we’ll start with the first ones we need to care about.

Step 1: The Mag Release

First, identify the spring. You’ll note that one is bigger than the rest of the assorted coil springs. This is the one we need now.

With that, you’ll need the lower in front of you and the following two parts:

  1. The mag release (that L-shaped guy with the threads)
  2. The mag release button

Insert the release mechanism on the lower receiver’s left side. There is only one place it fits, like.. you can’t miss it.

Once that bad boy is in there:

You’ll want to grip it so as not to let it fall out while you turn the lower over. Then slide that spring in the hole on the right side:

Once that’s in there, while maintaining that grip, take the mag release button and begin threading it on:

Go as far as it will let you.

*For these parts, keep spinning until it is about to scratch the lower, but stop and back off to the nearest settling position.

Then, flip the lower over, take something (my fingertip in this case), and push it against the mag release button itself.

Then take the mag release and start spinning it until you can’t anymore.

At this point, do a fit check. At rest, you should see the mag catch sticking out in the mag well:

And, once the mag release button is pressed (naturally), the mag catch should clear the mag well:

Step 2: The Bolt Release

Here, we have to identify a couple of things, and my picture-taking at this moment wasn’t the most intuitive.. but we’ll try to walk through it.

We need one of the shorter springs, they are different. And, one of the four detents, two of those are the same, and two are different. Of the different ones, one is actually called a plunger.

And one of the roll pins.

Let’s start with the springs and plunger.

We need that black mushroom tip plunger. But, as shown above, we have two very similar-looking small springs.

See that one on the right with a wide end? That’s not it. We need the straight and narrow one on the left.

We need the bolt release (pictured below) and a roll pin. But we have two roll pins, one big and one little. We need the little (pictured right below):

All the right parts together look like this:

And we’ll need our brass punch and hammer:

Now, I’ll take the roll pin and get it started. Just get her set with a finger push:

Then give her a little tappy-tap – but don’t let it get so far it winds up sticking into the gap.. just enough to hold it:

It should look about like this:

Then you want to take that spring and plunger, plunger facing out, and slide it in this hole:

Now, my pictures are a little out of order. This next one shows where the bolt release/catch goes in the lower (you’ll note the roll pin isn’t in these.. ignore that, please):

While in there, you’ll have to squeeze it with your fingers to get it in place:

Note above how I am also making sure the hole where the pin goes is lined up. But we already have the pin in place, so it is the same thing, except look at the other end to verify. This is while the whole thing is being kept in place with a secure grip.

And then.. grab the zip tie. This takes the place of a vise or the lack of another set of hands.

Loop it around all of this and just snug it up enough not to let the whole thing spring apart:

Then I flipped it around and played with the zip tie and placement until this all looked good and aligned:

Once happy, I finally sent her home:

Once set, she was seated and zip tie deleted:

Step 3: FCG (Fire Control Group)

We’ll start with the trigger.

This contains the trigger, the trigger spring, the disconnector, and that other little spring (with the fat end).

The trigger spring and the hammer spring have similar shapes, but one is thinner/smaller.

For the trigger, we want the thinner one (bottom left):

We need to set the trigger spring on the trigger as follows:

With that set. Take that last little spring with the fat end and shove it fat end down into the orifice on the top side of your trigger as follows:

With that set, you’ll prepare to lower the trigger assembly in place, as well as the disconnector:

You’ll place the trigger assembly into the lower. Note that the two sprung “legs” rest against the bottom, or floor, of the receiver:

Push it down into position to make sure it all settles correctly and your pinhole is lined up.

Now, put your disconnector in place on top of that little spring we placed in the trigger earlier and aligned with the pinholes:

It should look like this, resting:

Now, we need to pin it. There are two “pins,” one of which is called the “trigger pin.”

Sometimes, there is one with a single groove toward one end, the “trigger pin,” and one with a groove in the middle, the “hammer pin.”

You’ll note they are the same in this kit, so you can’t go wrong:

So, have the “right” pin at the ready, and push down on the entire trigger disconnector assembly until all the holes are in line:

Then take your “trigger pin” and start sliding it in:

You’ll need to wiggle, squirm, and adjust as you work it through.

And, after you wiggle enough, it should be all good to go:

Then, give ‘er a little feel, let ‘er know you love her (make sure there is spring pressure behind the trigger pull):

Finally, it’s hammer time.

Get your parts together. These are the hammer (shown below), the hammer spring (the thicker of the two we discussed earlier), and the “hammer pin” (as described in the previous section).

Follow the next two images to orient and install the hammer spring:

With that set, you’ll want to start inserting it into position.

Follow this series of images, note how we angle the legs into the lower and then set it down with finger pressure so everything lines up to that rear pinhole:

You can lock the hammer back, but it will still require you to keep everything in place with finger pressure. Doing this does help relieve some of that pressure you are providing:

Make sure the pinhole is aligned:

Then, wiggle the hammer pin into position until seated. You should be able to lock the hammer back into position at this time and have it hold itself:

While holding the hammer with your finger, so it doesn’t make contact during this test, pull the trigger and make sure all is working:

Step 4: Install The Safety and Grip

You’ll need the safety selector switch, the larger of the three detent springs, the larger of the detents (it has a flat end), the grip, and the screw that attaches the grip.

Install the safety selector switch first:

Then, place the lower topside facing down, take the detent, and install it (flat side up) in the hole located where the grip seats into the lower:

It should sit like this:

Now, pick up your grip and slide the detent spring into the hole located there (it corresponds with the location of the hole on the lower):

Using a finger to keep the spring in place, turn the grip over, align the spring with the hole containing your detent, and start pushing them together:

It should fully seat without issue:

Then insert and tighten the grip screw:

Now, verify the safety selector works.

With the selector in safe, you should not be able to pull the trigger:

With the selector in Fire, you should be able to release the hammer with the trigger (again, use your finger to hold the hammer back from making contact during this test):

Step 5: The Front Takedown Pin

This is one of those times when you may send a spring flying into the abyss; I’ll try to help you avoid that. This is where we’ll use that flathead pocket screwdriver and a pair of tweezers.

You’ll need the front takedown pin (it’s the one with the flat spot), a small detent spring, and the small detent:

With the front of the receiver oriented as shown, slide in the detent spring:

Here is the method I use to install the detent and pin.

I stand up the receiver on end and use the tweezers to hold the detent in place above the spring, then I take that flathead pocket screwdriver and start pushing down on the detent:

Once it starts going in the hole, I remove the tweezers and push the detent/spring combo all the way down:

This shot shows how skinny that flathead screwdriver is and how it clears the hole for the front takedown pin:

This allows you to now slide the front takedown pin into place, making sure the flat side of the takedown pin aligns with the flat spot on the receiver:

Once you get it in the hole, you can remove the screwdriver and send it the rest of the way:

Step 6: The Rear Takedown Pin & Buffer Tube

For this step, you’ll need the last of the small detent springs and detents, the buffer tube, the buffer retainer and spring (shown later), the end plate, and the castle nut (alongside a castle nut wrench and that pocket flathead screwdriver will become handy again):

First, go ahead and slide the rear takedown pin all the way in place (with the groove facing the rear of the lower):

With that fully inserted and the lower tilted forward, drop in the detent:

Then drop in the rear detent spring:

Next, gather up your buffer tube, end plate, castle nut, and buffer retainer and spring (assembled as shown):

Take the castle nut and go ahead and screw it onto the buffer tube as far as it will go (not tight, just stop when it makes contact with the last thread). Note the orientation of the castle nut:

With that seated all the way back, slide on your end plate (align the tab on the end plate with the groove in the buffer tube):

Now, start threading your buffer tube into the lower. Be mindful of the detent spring sticking out. Keeping the lower tilted forward will help keep it in place when you start threading on the buffer tube, and the spring will eventually contact the end plate. At that point, use your fingernail or something similar to push the spring in as you turn the buffer tube:

Make sure not to screw the buffer tube in so far that you can’t get the buffer retainer and spring in place; stopping around here:

At this point, you can install the buffer retainer spring and the retainer:

I used the pocket screwdriver to seat the rear takedown detent spring as I got toward the end.

You should also make sure the end plate seats flush as you get toward the final placement:

While making sure the groove in the buffer tube, made to capture the buffer retainer, lands in this alignment. You’ll need to push down the buffer retainer as you get to the last couple of turns that capture it:

Once here, and everything is being held in place and properly aligned, go ahead and tighten the castle nut with your castle nut wrench:

Next, you would take a steel punch and stake the castle nut in place by striking the end plate where it is aligned with one of these small grooves in the castle nut. (We did not actually do this as this thing is coming right back apart for another project):

Now you can slide in your buffer spring and buffer into your buffer tube:

Push it all the way until the buffer detent catches and holds the assembly.

Other Steps

Depending on your lower, the parts you have on hand, and your specific plans – you may or may not be installing these next two items at this time.

The Trigger Guard

Our trigger guard is pretty straightforward. It has a built-in detent at one end (the forward-facing end) and a hole for a roll pin on the other:

We go ahead and line up the pinhole in the rear of the trigger guard with the pinhole on the lower, insert the roll pin, and then use our brass punch to drive it in until fully seated:

After that is in place, we locked the front of the trigger guard in place:

The Stock

And we slid on the buttstock that came with the AR15 SOPMOD Buttstock Combo:

And that’s it.

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