When it comes to building an AR-15, there are so many different options, that it is somewhat difficult to narrow these options down to a comprehensive list. But we will go over the basics and what more operators choose for their first build, with other options tagged in, so there will be options for every operator and their particular case.
First you will need to purchase either a stripped lower receiver or an 80% lower receiver. Though, it is important to note that if you do go with the 80% lower receiver, you will have to also purchase a jig. There are websites that have these available in a package deal which will save you a couple bucks.
The jig is necessary for milling and drilling out the necessary holes that your AR-15 needs to function.
Though if you decide to go with a stripped lower receiver, one that is 100% complete, it will have a serial number and is consequently classified, by the ATF and GCA, as a legitimate firearm and requires the receiver to be shipped to an FFL location, and you will need to undergo the background check and incur transfer fees.
Either option for your lower receiver is fine, it is entirely up to personal choice.
Next, you will need a Lower Parts Kit, there are so many smaller parts that your AR-15 needs that it is much more time and cost efficient to just purchase a lower parts kit. It will save you the hassle of ensuring your got all the parts necessary.
A typical kit comes with 31 parts, including a pistol grip and trigger. But a lot of operators toss these in ‘save it bucket’ and go right for upgrades of these two parts.
Magpul makes a great grip for AR-15s that you can find directly on their website, or you can probably find it cheaper on Gun Broker or Cheaper Than Dirt.
Next as an upgrade to what is included in the lower parts kit a Fire Control Group or trigger, which consists of: hammer and hammer spring, trigger and trigger spring, disconnector and disconnector spring, and two fire control pins. This is an option for an operator who doesn’t like the trigger included in the kit.
Then you will need a Bolt Carrier Group. This is the bread and butter of what makes your AR-15 function and fire. The bolt carrier group includes the bolt carrier, complete bolt, firing pin, carrier pin and gas key.
The Buffer, which is the next part you need, slows down the bolt on the AR-15, absorbs recoil and reduces wear on the rifle. You also need a Buffer Tube and Buffer Spring.
Both of these parts come in different sizes depending on what type of rifle you have, so make sure you purchase the correct one.
Next, you will need a Barrel and Barrel Nut. Different barrels have different twist rates, length, and calibers, so do your research for what type of twist and caliber you want for your specific type of shooting you intend to do with your weapon.
A Gas Block and Gas Tube are necessary for your AR-15, it allows your weapon to cycle so another bullet can be fired.
Next, you need an Upper Receiver and an Upper Receiver Kit. The Upper receiver is basically the glue for your weapon, it ties everything together from the lower receiver and allows your weapon to shoot.
A Charging Handle, is next, and it moves the bolt carrier that is inside the upper receiver. Its primary function is to pull the bolt carrier back or to aid in clearing a malfunction.
Then finally, all you need are: Magazines, Handguards, and a Scope/Optics if you so choose.
It is an expensive task to build your own AR-15, but well worth it if you want to customize your weapon from out of the gate.