The folks at AT3 Tactical know a thing or two about building an AR-15. It is what we do. From the fit of fiddly detent springs to the final finish, AT3 is here to help.

Odds are, if you’re looking for some insight, you either want to build a gun from scratch or you want to modify an existing rifle. This guide will help with both.

Which parts work best? It depends on what you're building an AR-15 for.
Which parts work best? It depends on what you’re building an AR-15 for.

A bit of perspective on the AR-15

AR-15s, black rifles, modern sporting rifles… so many ways to parse the nomenclature. These guns trace their lineage back to Eugene Stoner’s original AR designs of the late 1950s. The Army was working with big, heavy rifles—notably the .308 M14—and they wanted something lighter and with higher capacity.

Stoner worked on the original AR-10, and later downsized to the AR-15. The AR stood for Armalite Rifle, but the design quickly moved into production at companies besides Armalite. The reason was simple—Stoner’s design was modular.

The AR-15 was designed to manufactured, easily assembled, and without a lot of expertise. You don’t have to be a gunsmith to build an AR-15. Anyone with a knowledge of the platform and the right set of parts and tools can do it.

Modularity means the parts can be swapped out without the need of a gunsmith.

Why is modularity important?

Modularity, at least in firearms, is a 20th century concept that came about from the need to have one type of gun that could be easily fixed in the field—something essential for military contracts. If you can swap out parts, any parts, without the need of a gunsmith, the guns are easier to keep in working order.

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So each part of a modular gun should—in theory—fit on any similar rifle. The 1911 was designed like this, but anyone who knows the guts of a 1911 knows this isn’t really true. But the AR-15 was different.

An AR-15, in its truest form, is like open-source hardware. You should be able to take the upper off any AR-15 and put it on the lower of any AR-15. Matching calibers is a concern, but everything should fit and function without the need of permanent modification. This is what makes building an AR-15 so easy.

You’ll learn much more by building a rifle yourself than you would just by shooting it.

Do It Yourself

That’s what makes the platform so attractive to the DIY crowd. You can buy a new rifle (or a pistol AR) and start swapping out parts as you see fit (as long as you stay within legal boundaries of pistols and short-barreled rifles). Or you can begin from scratch and start picking out the parts you want from the beginning.

This modularity wasn’t designed to feed end-users’ desires to make their own custom guns, but that was one result. What began as a rifle to fit the unique needs of the Army has become a gun platform that has been adapted for target shooting, self-defense, hunting and more. The AR is one of the most customizable and adaptable guns ever made.

Why build your own AR?

Why not? If ammo prices are through the roof, building an AR-15 or modifying a rifle is a great way to stay in touch with your hobby without running money down-range with every pull of the trigger. Get in there and see how the AR works, firsthand.

Customizing your gun can take a functional gun to the next level. This is an AT3 Tactical upper in FDE.
Customizing your gun can take a functional gun to the next level. This is an AT3 Tactical upper in FDE.

And there’s a pride of doing it yourself, too. You can buy an off-the-shelf gun that embodies a manufacturer’s vision of what they think you want—nothing wrong with that. Or you can step outside of their preconceived notions about you and design your own.

Afterall, many AR parts used by big name brands (and small ones, too) are made by the same suppliers. Going to a major manufacturer will allow you to buy a gun that is branded, but a lot of the forgings for uppers and lowers, as well as barrels, are coming from big behind-the-scenes parts manufacturers that supply multiple manufacturers with the exact same raw materials.

This speaks to the strengths of modularity that were mentioned earlier. It also suggests that some of what you pay for at boutique manufacturers is the branding and not the inherent quality of the parts. But that’s a personal decision.

Building an AR-15 from Scratch

One good reason for building your own gun is the ability to get it right the first time. While the AR-15 began as a line of guns that looked as indistinguishable as the soldiers that carried them, it has become a platform for self-expression. If you can’t find what you want off the shelf, you have to build it.


Many of us also build multiple uppers for our guns. Long, heavy barrels are great for shooting long distances. Thinner, shorter barrels are ideal for maneuverability. You may only need one lower to accommodate both. You may only need one lower to accommodate multiple calibers.

The lower is the only serialized part of the gun and has to be transferred through an FFL.
The lower is the only serialized part of the gun and has to be transferred through an FFL.

And maybe you’d rather have some unique finishes or paint or want to build a retro classic. The modification is really part of a journey of sorts, and you’ll know when you get there. Building that perfect gun that fits you and your needs feels really good.

Building an AR-15 on a Budget

Many of us jump into this as a hobby, though, because we want (or need) to save some money. Building an AR may seem like a solid way to put together a gun on the cheap, but the costs do tend to add up. If the end-cost is important, price all the parts before you get started.

This is a solid way, though, to spread cost out over time. Work on the lower first, and then—maybe a month or so down the road—pick up the parts for the upper. Later, add the furniture.

Customizing the AR you Already Own

Buying a complete gun off the shelf may cost less than building an AR-15 on your own, depending on the cost of the complete rifle and the cost of your parts, but this isn’t always the case. Big manufacturers have buying power, so can sell guns for less when they source parts in bulk. But large parts specialists (like can also use that buying power to offer some solid deals on parts.

And consider the cost of buying a complete gun and then having to replace parts–like triggers—that can get expensive, too. There’s nothing wrong with the approach. Many of us began tinkering with ARs by buying entry level guns that are fully functional out of the box and slowly refining elements like triggers, furniture and barrels.

This style of DIY is especially popular in times when gun companies can’t keep up with demand. Working on the gun you own will keep you in the game.

The Real Avid Gun Tool Pro is a good start for anyone wanting to start building (or even cleaning) AR-15s.
The Real Avid Gun Tool Pro is a good start for anyone wanting to start building (or even cleaning) AR-15s.

Tools for building an AR-15

As do-it-yourself projects go, anyone with advanced assembly skills can put together an AR-15. It can be done with a very simple set of basic tools, though the specialized armorer’s tools and clamps make the process easier. But if you are in any way handy, this is a project you will not struggle with.

And building a gun will provide you with solid insight into how that gun works. Once you have held all the springs of the trigger group in place and pinned the whole shebang into a lower, you will begin to get the bigger cause-and-effect relationship picture down.

The benefit here is obvious. If you’re on the range or in the field and something goes south, you will have the skills to diagnose the problem and, potentially, to fix it. Throw a lower parts kit in your toolbox and you’re good-to-go.

But there’s more to learn, too. We have a solid video series. YouTube is a great resource, but there are fantastic articles on the subject and even some trusted (if somewhat old fashioned) books on the subject. You’ll get a crash course in the science of headspace and torque, but it is worth it.

Learning at this pace and level is easy. And it is fascinating for most of us. What’s the difference between a carbine length gas system and a mid-length gas system? You’ll know that answer and many other. If you have read this far into this article, you are one of us.

AT3 has just about everything you'll need to build a rifle from scratch.
AT3 has just about everything you’ll need to build a rifle from scratch.

Stay tuned

While much of what we’re talking here is motivation, there’s a lot more to parse out. As we dive in, this library will grow. Let us know what you need to know, and we’ll find the answers.

One Last Tip

If there’s anyone that knows the AR-15 platform, it’s the US military. As a special offer for our readers, you can get the Official US Army Manual for AR-15/M4/M16 right now – for free. Click here to snag a copy.

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2 thoughts on “Your Guide to Building an AR-15

  1. I take exception to your note below the lower unit picture above, namely “The lower is the only serialized part of the gun and has to be transferred through an FFL”. that is true for a 100% completed, stripped lower. although, a lower at the 80% complete stage, and defined as such (80% lower) does not require an FFL transfer or sale.

  2. Do you have manuals for spring operated A.R. 15‘s like Adams arms? If not where can I get one?

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