What are the parts of an AR-15?

All over the internet, at least the parts that have to do with firearms, there is information about building your own AR-15. The problem I found when researching this is no one ever says what exactly each part is and what it contains. There are different part kits, build kits, rifle kits, etc. Below is a fairly in-depth guide to all of the components of an AR-15 and how they are commonly packaged for sale. I will be skipping over all of the tiny parts such as functioning springs, rings, etc. Some parts will also simplify how the firearm actually operates for simplicity sake.

There are 2 main “parts” to an AR that are build of sub-components: the Lower and the Upper. The full name for these are upper / lower receiver.

A completed upper consists of:

  • The actual upper receiver. By itself, it is commonly referred to as a stripped upper. The stripped upper has some options that can come with it.
    • Forward assist. This ‘button’ will force the bolt forward. That is to say that it will push an ‘out of battery’ bolt the rest of the way closed. Will not close the bolt if held by the bolt catch
    • Dust cover. This is a little springed door that you can close to prevent shit from getting in the ejection port. It automatically pops down when the bolt moves.
  • The Barrel. More often than not, this will have the front sight attached. The barrel has rifling through the core, in a specific ratio. This will be either 1:7, 1:8, or 1:9. 1:7 is rifled for lighter weight bullets, 1:9 for heavier bullets and 1:8 for middle of the two. They also come in different linings that offer different levels of corrosion protection.
  • The gas block. The gas block’s purpose is to take exhaust / pressure from the firing and route it back to the bolt.
  • The hand guard. This is the heat shielding around the barrel that allows you to hold the AR without searing your precious skin off.
  • The bolt carrier group. This is the part that pushes a new round into the chamber, pulls the spent cartridge out and slides back and forth.
    • The bolt. The main component of the bolt carrier group.
    • The firing pin. This is the part that hits the primer of the cartridge, firing the bullet.
    • The extractor. This part pulls the spent casing from the barrel.
    • The ejector. This part ejects the spend casing from the ejection port
    • The BC assembly. This is the housing that all of the BCG parts sit in
    • The bolt carrier key. This allows the gasses from the gas block to push the bolt back
  • The charging handle. This is the part behind the bolt that you pull on to pull the bolt back.
  • Muzzle break. The muzzle break sits at the end of the barrel and dictates the flow of heated gasses coming out of the barrel. Special ones will direct the gas in a certain direction, hide the flash of burning cordite, and other things.
  • Sights. Sights are composed of the rear and front sight. Note: The inclusion of the rear sight in any kit is hit or miss. A completed rifle should have one, but any kits below that refer to this section do not automatically include a sight. Make sure to check.

A completed lower consists of:

  • The actual lower receiver. This is the main housing that all of the parts connect to and is considered “the gun”. This is the part that must be sent to an FFL if you are receiving by mail. All other parts may come directly to your door. Any set of parts containing the lower must be sent to an FFL. By itself, it is referred to as a stripped lower receiver.
    • (Optional) 80% lower receiver. This is just like the stripped lower above, but it has not been fully machined. Holes and ports must still be drilled out or cut out. Not considered a complete stripped lower and does not require transfer to an FFL. By completing the lower yourself, you become the manufacturer.
  • The buffer assembly. The function of this is to be hit by the bolt and absorb some of the recoil through a spring. Has sub-components of : Buffer, buffer tube, buffer spring.
  • The stock. The stock sits around the buffer tube and goes up against your shoulder. Some move, some don’t, some are spring loaded. It’s all about what you like.
  • The fire control group. This part is the trigger, and all of the stuff that makes a trigger work. This has parts such as the trigger/sear, and the hammer.
  • The grip. Most common grip for an AR is the pistol grip. It comes down, away from the rifle and allows you to hold the rifle in your hand similar to a pistol.

Parts not mentioned that you may run into:

  • Rails. These go along the barrel and on top of the rifle. They allow for attachments to be attached to the rifle.
  • The magazine. This part holds the cartridges and feeds into the bottom of the lower. IT IS NOT A CLIP.
  • The clip. A device (NOT THE MAGAZINE) that allows for quick loading of a magazine.

Common groupings of these parts:

  • A completed Lower: All of the parts mentioned in the Lower section
  • A completed Upper: Also referred to as a barreled upper assembly. All of the parts mentioned in the upper guide WITH ONE CAVEAT: The BCG and charging handle may or may not be included. Make sure to look for this.
  • A lower build kit: All of the parts mentioned in the lower section, minus the stripped lower.
  • A lower PARTS kit: All of the parts in the lower build kit, minus the stock and buffer assembly.
  • Rifle kit: Everything you need to build an AR, minus the stripped lower.
  • I hope this guide has cleared some of your confusion surrounding the building of an AR-15.

    Updates to come as I find my errors or find new information. Current as of April 27, 2016.