The Beginner’s Guide to Building an AR-15 Part One

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M4A1 (AR-15) tactical carbine on the bulletproof vest

AR-15s are extremely versatile firearms, and you can customize them in nearly an infinite number of ways.

I almost have more fun building AR-15s than I do shooting or hunting with them.

Yes, an AR-15 can be a great hunting rifle!

However, this versatility can be overwhelming when you’re first getting into AR-15s. I spent a year researching and ordering parts for my first AR.

This article will help AR newbies and even some veterans think about building their first AR-15. It’s not a nitty-gritty technical guide, but it can help you understand why you should go with certain options and how they all go together. Overall, this guide should be a good starting place for further research.

It’s also part one of a two-part series. The second part will cover how to put your AR-15 together.

AR-15 Lower and Upper

AR-15s are modular guns. They consist of two major parts:

  • The upper
  • The lower

Let’s look at those in detail.

 

The Lower

man holding Palmetto State Armory AR15 lower receiver

The bottom half of the receiver is often called the “lower” or “lower receiver.” In the US, this is the part that’s legally the firearm, and it will have a serial number.

Every AR-15 part except for the lower can be shipped to your door. You’ll need to buy a lower from your local gun store or have one shipped to someone who holds a Federal Firearms License.

You can also manufacture your own AR-15 lower from an 80% receiver, but that’s outside the scope of this article.

The lower receiver holds all of the fire control group (FCG) components. These are the trigger, hammer, sear, and all related springs.

The grip and stock also attach to the lower receiver. Plus, there are additional controls: the bolt catch and magazine release. There’s also the buffer tube.

 

The Upper

AR-15 upper disassembled on table

The upper receiver is where the gun goes “bang!”

It holds together the barrel and bolt carrier group (BCG). The BCG contains the bolt, bolt carrier, gas key, and firing pin.

Attached to the front of the upper is the handguard. Between that and the barrel is the gas tube, which connects to a gas block partway down the barrel. At the end is a muzzle device—most of the time, at least.

Controls on the upper are limited to the charging handle and the forward assist.

On top of the receiver are whatever sights and optics you choose to attach.

The upper isn’t serialized, and you can remove it from the lower by depressing two pins, so you can use any number of uppers with one lower receiver.

 

How To Build An AR-15

1. What To Think About

man holding rifle in the forest

The first thing you need to do is determine what you want out of your AR-15. This will greatly influence which parts you need.

Do you want to plink with it? Engage in long-range marksmanship? Hunt? Defend your house? Clone a cool or historic military rifle like the XM16E1 or Mk18?

 

Choose the Chambering

Round in chamber of gun

Most AR-15s are chambered in .223 Remington, 5.56×45 NATO, or .223 Wylde. There’s not much difference between all of those, but the specs are slightly different.

5.56 is slightly hotter than .223 Rem, so you don’t want to shoot mil-spec 5.56 ammo out of an AR chambered in .223 Rem. .223 Wylde is supposed to be halfway between the two and is a good compromise.

5.56/.223 work best out of a 20-inch barrel. The shorter you go, the more unburnt powder ejects out the front of the barrel, and the less velocity you’ll get. That velocity loss starts to nosedive when you get shorter than 11.5 inches, which is the shortest I’d go against anything but paper.

Twist rate isn’t super important unless you’re building a long-range precision rifle. Avoid 1:12 and you’ll be fine. Between 1:7 and 1:9 are all good for most purposes.

If you want to build an AR for a certain purpose, then you’ll want it chambered in a specific cartridge. I like .300 Blackout for short-range hunting and 6.5 Grendel for long-range hunting and precision shooting.

 

Think About Weight

both hands holding AR-15 up the sky

I must admit, the first AR-15 I held was not to my liking. At all. It was cumbersome, awkward to use, and turned me off the platform for a long time.

That’s because that AR had a 20-inch HBAR barrel with a carbine buttstock. It was unbalanced and only good for bench shooting.

My first AR was much better in my hand. It had an 18-inch, mid-weight barrel and a free-floating handguard without rails. Much easier to use, though still not as spry as some of my later builds.

Like most firearms, ARs made for bench shooting tend to be heavier than ARs made for competitive shooting or hunting.

Keep in mind where you’re going to use the gun. Adjustable precision stocks are great but heavy. HBAR barrels are stiffer than lightweight barrels and will be more precise, but they will slow you down.

Three-Gun AR-15s tend to be halfway between benchrest and ultralight guns. You’re shooting them offhand and need to be able to quickly point with them, but some weight does help mitigate recoil and prevent muzzle rise.

 

Keep It Simple

hand assembling rifle lower receiver

It can be tempting to have your first AR do everything.

I’ve seen too many first-time builders get a quad rail and add a vertical grip, flashlight, laser, chainsaw, and kitchen sink.

Please don’t fall for this trap. Those all weigh your gun down and add complexity, which makes your gun less fun to use.

Every AR part is replaceable. Start simple and light, see what you like, and upgrade from there. Don’t jump all in with every nifty feature you can find.

 

Gas System Length

hand picking a rifle

AR-15s are direct-impingement firearms, so there’s a hole in the barrel which taps into the expanding gasses to work the action.

There are four standard gas system lengths: rifle, mid-length, carbine, and pistol.

Pistol gas tubes are 4.5 inches long, carbine tubes are 7.5 inches long, mid-length tubes are 9.5 inches long, and rifle tubes are 13 inches long.

The longer the gas system, the gentler the recoil impulse, which makes shooting the rifle more pleasant. Shorter gas systems hit your BCG faster, causing more wear as well.

It’s generally recommended to use the longest gas system length you can for your chosen barrel length if your barrel is chambered in 5.56/.223. Other cartridges differ. For example, .300 Blackout almost always prefers a pistol-length gas system even with a longer barrel.

 

Rifle or Pistol?

man holding AR-15 pistol

AR pistols are very popular right now. They are quick and light, and almost as effective as a rifle.

Rifles have a minimum barrel length of 16 inches and a minimum overall length of 26 inches. Pistols, however, have no minimum length. You can’t put a stock or vertical foregrip on a pistol, though.

What makes AR pistols so popular is that you can put an arm brace on them instead of a stock. When this article was written, the ATF allowed shooters to shoulder a pistol with a brace.

Putting a stock on a short barrel, however, turns your gun into an SBR. This is a felony unless you’ve registered the firearm with the ATF using a Form 1, paid the $200 tax stamp, and received approval.

 

2. Necessary Tools

man assembling rifle with proper tools

You don’t need very many tools to build an AR-15. I’ve done it in a basement without half these tools before.

It’s much easier to build an AR-15 with the proper tools, though!

Vise Blocks: You want a vise block for both your upper and lower. These will securely hold the receiver and help prevent damage. You can crush an upper if you just stick it into a vise without a block!

Punch Set: AR-15s have multiple pins, including a couple of roll pins. Can you insert roll pins without a roll pin punch? Yes, but there will be much wailing and gnashing of teeth.

Armorer’s Wrench: A good armorer’s wrench will help you install the barrel nut, castle nut, and muzzle device.

Torque Wrench: This is good for making sure your barrel nut is on tightly enough so your barrel doesn’t come loose!

Grease: This is also used for the barrel nut. Molybdenum or white lithium is preferred. Anti-seize also works, provided it contains no graphite. Graphite will corrode the aluminum receiver!

Misc Tools: Depending on the parts you choose, you may need screwdrivers, Allen wrenches, or other small tools. Chances are you have these already or they came with your parts.

 

2. Necessary Parts

hands assembling rifle bolt

Here I’ll go over all the parts you need to build your first AR-15, along with some recommendations for each.

By the way, you don’t have to assemble an AR completely from stripped parts. It’s perfectly okay to buy a complete upper and a complete lower, and attach them together.

 

Lower Receiver

rifle buffer and stock assembled

While there are some upgraded lower receivers out there, 95 percent of them will be mil-spec aluminum lowers. Stick with this for your first build and branch into other types later.

Lower parts kit: A mil-spec LPK will contain all the small parts you need to finish the lower, including pins, bolt and magazine catches, safety selector, trigger guard, etc. If this is your first rodeo, stick with mil-spec.

Fire control group: This is the trigger, sear, and hammer. Most LPKs come with FCGs, but you can get them separately.  Mil-spec triggers aren’t that great, so this is a good day-one upgrade. Drop-in FCG modules are easier to install but often require using anti-walk pins.

Pistol grip: Every AR builder has a drawer full of A2 pistol grips because they come with the LPK. Some people (weirdos) like them. I prefer Magpul K2+ grips. Try some and see what you like.

Buttstock: Almost as important as the grip for ergonomics is the buttstock. You can spend almost nothing or several hundred dollars on the stock. Try some out at the store and see what works best for you. The Magpul MOE is a good, inexpensive stock.

Buffer tube and buffer assembly: These typically come as a complete kit, sometimes with a stock. A carbine buffer works for most AR-15 builds.

 

Upper Receiver

disassembled upper receiver of rifle

The upper half of your gun and no less important than the lower.

Barrel: A good barrel makes or breaks your rifle. Cheap barrels are good for blasting guns, but if you want precision, pay more for a quality barrel.

Bolt carrier group: While you can buy the bolt, carrier, and other parts separately, it’s best to buy a complete BCG. Make sure it’s been MPI tested and has a staked gas key. Toolcraft BCGs are reliable and inexpensive. Some BCGs have fancy coatings, but they all work well when oiled, so you don’t need to pay the expense.

Charging handle: While beginners will be happy with mil-spec components most of the time, I highly recommend paying for an ambidextrous charging handle. It makes your AR-15 so much easier to use.

Upper receiver parts kit: This covers the dust cover and forward assist.

Gas tube: There’s no difference between a cheap and an expensive gas tube.

Gas block: For beginners, I’d recommend a gas block that attaches via set screws. They can have (extremely) minor effects on accuracy but are also easy to install.

Muzzle device: A flash hider is cheap but won’t do much to affect recoil. Muzzle breaks mitigate recoil and help keep the muzzle still but will hit people next to you with a concussive wave. Linear compensators push the blast forward and help with muzzle control but won’t do anything for recoil.

Handguard: Unless you want your AR-15 to be a clone, I recommend going with a free-floating M-Lok handguard that’s long enough to cover the gas block. You’ll get customizability without weight.

Sights and optics: This is a personal preference and heavily depends on how you’ll use the gun. I’d recommend starting with iron sights if this is your first gun, though.

If you insist on getting on then red dots are a popular choice.

 

Conclusion

gun wall rack with rifles

Choosing all of the parts for your first AR-15 build can lead to decision fatigue. My advice is to take your time looking around and read other people’s opinions on certain parts.

Once you know how you want to build your gun, then you should start buying the parts. I generally wait until there’s a good sale before buying handfuls of parts. This is why it took a year to build my first AR, though. There’s no shame in getting all the parts together quickly!

Once you have everything, then join us for part two.

Actually, why don’t you go ahead and read that now? That way, you know what you’re getting into!

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Andrew Jackson learned to walk in the mountains and has spent much of his life exploring the outdoors. He is equally at home in the woods, at the range, or on the gunsmithing bench, and loves to build guns almost as much as he enjoys shooting them. His travels have taken him to the four corners of the United States. Though his favorite hunting spot is in Alaska, Kansas deer taste better.

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