Parts Needed to Build an AR-15

The AR-15 is the most popular rifle in the US, and has been for a few years according to sales rankings.  It's popular for many reasons as I've written about in other posts.  It's flexible, available in a variety of calibers rainging from rimfire to 50 caliber.  It's a great choice for home defense.  It's the most popular choice for government agencies.  With a collapsing stock the same rifle can fit small and large shooters.  In the most common chambering of 5.56 NATO or .223 Remington it offers enough power for small and medium game hunting and light recoil since it's an intermediate power cartridge.   223 is among the most affordable centerfire rifle rounds, at least when we aren't in the middle of the great ammo panic of 2013.  Quality magazines can be had for $10 each.  All of these factors have made it the most popular rifle today.

Building an AR-15 has become quite popular.  It doesn't involve any real gunsmithing because you're just assembling parts.  There is no hand fitting needed, it's almost lego-like.  But if you want to put together something specific or save some money on a quality rifle, assembling yourself could be the way to go.

To assemble a lower you only need one special tool, a wrench to snug down the castle nut if you are using a collapsing stock. You can even get by without that by using a steady hand with a hammer and a screwdriver.  To assemble an upper you need a couple other tools I will discuss when we get to that section, but uppers that are already assembled are widely available in many configurations and are pretty affordable.

A typical stripped lower receiver

The part of the AR15 that is the “firearm” according to the BATFE is the lower receiver. This is the only part you will need to purchase or have transferred to at your local FFL. Keep in mind if you buy one online you'll have to pay the purchase price plus a shipping cost plus a transfer fee at your local FFL.  It's often cheaper to buy one your dealer has in stock.  If you want some information on the details of different brands of lower receivers you can find that here. In general, the lower receiver doesn't need to be from a premium maker, it just needs to be in spec. Prices in shops start around $120, you can spend more on one that's fancier or from a premium name.

After your lower is purchased, you'll need the following items to complete your rifle:

Lower Parts Kit (commonly called the LPK)

Stock and Stock Kit

Barreled Upper Receiver or individual parts

Sights and/or Optic

A LPK is a standard offering that contains all the parts to complete your lower. It's comprised of a variety of pins, springs, buttons, the trigger assembly and the grip.  If you buy it from a reputable manufacturer it should work perfectly. The difference between cheap LPK's, like a DPMS, and ones that cost more, like Rock River, can be felt in the trigger.  Other than the trigger, parts are parts.  A nicely finished trigger makes a rifle easier to shoot accurately so it may be worth it to spring for a LPK with a nice trigger.  Some LPK's are offered without trigger assemblies allowing you to pick a trigger for yourself.  The ALG offerings are a very nice option that's pretty affordable.

Another thing to consider the grip.  Most LPK's come with a standard black A2 grip. It's hard an plasticky and kinda suck.  There are options available that offer a better fit for the hand, or an enclosed storage space or an angle that's more comfortable for the wrist.  You can sometimes find LPK's that have other grips.  If it's a part you want to replace later, it's easy enough to change out the grip later on.

One part people can forget when building out their AR-15 is the buffer tube (also called the receiver extension). This part attaches to the rear of the lower receiver and it contains the buffer spring and buffer (which you will also need unless you purchase a stock kit that includes them) and the stock slides onto it. You can purchase a stock kit that contains the Stock, buffer tube, buffer spring, buffer, endplate and castle nutor you can buy the parts separately.  There are a couple things to consider when choosing these parts.  Primarily is the stock, if you want a stock that has storage or other features this is the time to do it.  You can get a standard carbine buffer, or you may find a heavy buffer works better for your application.  You can read about buffers and springs here, here and here.  You don't need to get too worked up, a carbine buffer that comes in a kit is going to work just fine.

Another item some people change out is the endplate.  You can get an endplate that offers a stock attachment if that wish to run a single point stock.  There are a number of choices you can use.  This part is best installed during the initial build, it can be a bit of a pain to swap out later.

Most people buy barreled uppers that are already put together. If you do this you don't need to buy the special tools to assemble your own. (a barrel nut wrench and a upper receiver block) If you have access to those tools or are planning a couple builds you can save money by assembling your upper yourself.  If not, it's likely more affordable to buy your upper already built.  You can commonly find uppers that have whatever handguard you may choose or uppers that don't have handguards if you want to install your own. Keep in mind to install many free float handguards you need to remove your barrel nut and delta ring and will need the above mentioned tools.

The AR-15 directs gas from the barrel, through a tube that sits under the handguard, to the bolt carrier to operate the firearm.  There is a hole of a very specific size drilled in the top of the barrel.  The gas block seals around this hole and connects it do the gas tube.  There are 5 different places where a gas port is drilled. The shortest one is pistol length, typically only found on very short (pistol) barrels and special builds.  The next shortest and most common is carbine length- this is the original length gas system.  Becoming more common is a bit longer, called mid-length.  If you are building a carbine, you probably want a mid-length.  Read up on it if you like.  On longer barrels (18" and 20") there are longer gas systems, intermediate length and rifle length.

Why do you care?  Many times your front sight is part of your gas block, so your handguard will have to match that length.  If you have a carbine length gas system, you'll want carbine handguards.  If you have a mid-length gas system, you'll need the longer mid-length hardguards.  If you have a low profile gas block, you can run any length handguard over it, but keep in mind you'll need to purchase a front sight then.  Of course, if you buy an upper that's already assembled then you don't have to worry about it. 

Lastly, some upper receivers come with the bolt, bolt carrier group (called BCG) and charging handle.  Some do not.  If the assembled upper does not include these parts you will need them.

If you decide to built your own upper, the parts you need to build your upper are:

Upper Receiver (stripped)

Trap Door Assembly(Optional)

Forward Assist Assembly (Optional)

Barrel

Barrel Nut (sometimes included with free float handguards)

Delta Ring (if you are using a standard or MOE handguard)

Barrel

Gas Block (sized appropriately for your barrel, in low profile, with a sight block or railed to install a sight)

Gas Tube (for the appropriate length for your barrel gas port)

Muzzle Attachment (muzzle brake or flash hider)

If you want a specific barrel, barrel attachment and/or handguard, it may be best to build your own. If you want a pretty standard offering, it may be best to buy one already assembled.

So what's left?  A few things.  An optic and/or a rear sight.  A front sight if you went with a gas block that doesn't include a sight.  A magazine or 5.  A sling.  A case to hold it all.  Lots and lots of ammo!

 Here is an example of a build using high end parts:
Your choice of lower, $120
Bravo Company 16" mid-length assembled upper receiver, $460
Black mid-length MOE handguard, $35
Magpul MOE stock kit, $60
Spikes Bolt Carrier Group, $140
PSA LPK with MOE grip, $60
2 point sling, $20
BCM Gunfighter (small) charging handle, $45
Flip-up rear sight, $55
10 Magazines, $90
That puts your total at just over $1,000.  If you wait for sales you can knock 10% or more off that total.  If you choose other options you can spend hundreds more or hundreds less.

As an example, you can go with a basic setup like this:
Your choice of lower, $120
Palmetto State Armory Carbine Kit, $500
Fixed rear sight, $30
Gen 2 P-Mag, $15
That totals up to under $700 and gives you a functional and reliable rifle.

If you want to find parts in stock, this thread on AR15.com is a good place to start.

Good luck!