# AR-15 Parts Explained: Diagram, Names & How It Works Every AR-15 is roughly the same rifle underneath. Learn the parts once and every spec sheet, upgrade thread, and gun-counter conversation starts making sense. This guide **walks through the whole rifle**, upper to lower, with a labeled diagram and a plain explanation of what each part does, why it is built the way it is, and which specs matter when you compare two rifles. I have been shooting, building, and breaking AR-15s for over a decade and the Pew Pew Tactical team has put well over 100,000 rounds through them. If you already know the parts and want to know which rifle to buy, start with our [**Best AR-15 Picks**](https://www.pewpewtactical.com/best-ar-15/). If you are building one, the **[AR-15 parts list and tools list](https://www.pewpewtactical.com/ar-15-parts-list/)** has every item you need. Everyone else...read on! AR-15 Major Parts Diagram ------------------------- The AR-15 consists primarily of two large segments: the lower receiver... And the upper receiver, which can be swapped out in a matter of seconds. The AR-15 can fill multiple roles, ranging from long-range precision rifles to close-quarters combat (CQB) carbines, by simply switching out the "upper" and mating it with the "lower." **Only the lower** **is the serialized firearm**, so with one lower, you can have several uppers to fulfill any role without additional paperwork. The lower holds the trigger mechanism, and for the most part, you can easily swap out the uppers. You can even swap over from 5.56 to different calibers, like .22LR, .300 Blackout, .50 Beowulf, and more by simply tossing on a new upper. Let's start with the upper... All About AR Barrels -------------------- We begin the breakdown of the AR-15 system by covering the various characteristics of barrels (the thing that **[bullets ](https://www.pewpewtactical.com/how-guns-work/)**come out of). Now the real fun is going to start! ### Barrel Chambering The barrel chamber means the part of the barrel where the cartridge sits before firing (ie, what ammo it can shoot). Since this is a beginner's guide, we'll focus on the **most common** 5.56x45mm NATO and .223 Remington. There are tons of [**other chamberings**](https://www.pewpewtactical.com/best-alternative-ar-15-cartridges-and-calibers/) from .22LR to .50 Beowulf, and a slew of new .30 caliber offerings. The 5.56 is the more common choice compared to .223 since it gives "looser" tolerances and can withstand the higher pressures of the 5.56 round. Here are some general safety guidelines for using different cartridges in different barrels: - With a 5.56 barrel, you can shoot **BOTH** 5.56 and .223 - With a .223 barrel, you can **ONLY** shoot .223 - Hybrid chambers, such as .223 Wylde, are for specific precision purposes but can shoot both ### Barrel Length: Long & Strong Federal law states that the minimum barrel length of a rifle must be at least 16 inches. This includes muzzle devices, such as flash hiders, muzzle brakes, or compensators. However, if the device is to be included in the length, it must be permanently attached. For example, you can have a 14.5-inch barrel and permanently pin/weld a muzzle device to make it over 16 inches to make it compliant. Please note that there are some state and local laws that may govern the types of devices allowed, and even having a 14.5-inch barrel might be illegal without the appropriate paperwork. Therefore, we recommend beginners get **at least a 16-inch barrel** so they can switch out muzzle devices to their liking. Most common lengths include 16-, 18-, and 20-inches. So, which one do you choose? Having a longer barrel doesn't necessarily mean more accuracy. You can get plenty of accuracy out of 16 inches since it is shorter and therefore **stiffer** and less susceptible to barrel whip (movement of the barrel during shooting). However, **a longer barrel gives you** **higher velocity** since there's more room for all the powder to burn. And faster-moving bullets give the environment (gravity and wind) less time to affect the trajectory of the bullet. The average 16-inch barrel is good for up to 400 yards, but after 300 yards, the standard 55-grain projectile becomes more susceptible to environmental factors. For longer distances, you would want a heavier and longer projectile such as 62gr, 77gr, or 80gr bullets. Lastly, there's also the issue of portability. Shorter barrels are easier to move and weigh less. So many things to think about! ### Barrel Twist Rate Another number you'll see when looking at barrels is the twist rate of the rifling. This is shown as the number one, followed by a colon, and then another number (example - 1:9). A 1:9 twist rate would mean the bullet makes one full rotation (twist) every 9 inches. In general, the longer a projectile is, the faster the twist is needed. The smaller the second number, the faster the twist rate. Below is a great chart to show you the ideal bullet weight for the twist of a barrel. The most common AR-15 twist rate is now 1:7 or 1:8 since 55gr is the most commercially available, while the most common mil-spec twist is 1:7 since they need to stabilize longer/heavier tracer rounds. ### Barrel Material There are a lot of numbers and scary-sounding words out there when related to barrel material, and we've done our best to simplify it: - **4150**: Steel used in mil-spec barrels - **4140**: Steel with 10% less carbon than 4150 - **Chrome Molybdenum Vanadium, Chrome Moly, or CMV**: Same as 4140 - **Stainless Steel**: More accurate but shorter lifespan For the average shooter, you will be fine with 4140 CMV barrels. Unless you're firing full-auto a lot, you probably would not reap any benefit from 4150, except for a lighter wallet, but that probably doesn't count as a benefit. ### Barrel Lining - **Chrome Lined**: Coating that makes your barrel last longer at the price of a slight decrease in accuracy. You'll see a gray ring around each end. - **Ferritic Nitrocarburizing (FNC)**: Also known as Tenifer, Melonite, or Nitride. Treats the surface of the barrel instead of a coating for possibly better accuracy, but with additional cost. - **None**: No coating. Real round counts will differ based on a lot of factors (heat, environment, your definition of "accuracy," etc), but you can expect around 10-20,000 rounds before you have to re-barrel. You still with me? ### Barrel Testing Good, here's some more acronyms manufacturers will throw at you. - **MP**: Magnetic Particle tested, magnetizing the barrel and using fine iron particles to detect cracks/defects - **HP**: High Pressure tested - **None**: No testing. You get what you pay for. You might run into the use of "batch tested," which really doesn't mean much. The manufacturer may test 1 out of 10 barrels or 1 out of 100,000 barrels. For a home defense rifle, we recommend getting a barrel that is both MP & HP tested. ### Barrel Forging - **Cold, Hammer, Forged (CHF)**: Process that creates a more durable barrel - **Barrel, Forged, Hammer (BFH)**: Same as CHF - **None**: No extra process CHF and BFH reduce accuracy a little, but you gain a **much more durable barrel**. If you plan on shooting thousands of rounds a year, it might be worth the extra cost and push you towards a 20,000+ round count. ### Barrel Contours The shape and thickness of the barrel will have a decent effect on the overall weight. And if you haven't seen this quote yet on forums, **"ounces equal pounds and pounds equal pain."** - **Heavy (sometimes called bull barrel)**: Stiffer and heavier but more accurate. Can soak up a lot of heat before shooting groups start to suffer. Normally used for precision builds. .936-inches in diameter. - **Medium (government or M4)**: All around balance. M4 contour has a cutout for grenade launchers. .750-inches in diameter. - **Light (sometimes referred to as pencil)**: Lighter but more susceptible to heat from rapid firing. .625-inches in diameter. For the average plinker, medium or light barrel contours will work great. Many manufacturers also have their own custom profiles or specific names. They will still usually denote what general contour they are (Heavy, Medium, Light). ### Barrel Feed Ramps This is an integral part of the upper receiver and you want it to be a proper match with your barrel. The jury is still out whether or not they help reliability, but the main thing is to match up the ramps with your receiver. If you're buying from the factory, this should not be a problem. But you still might want to check. Curious what can befall ARs? Check out **[Most Common AR-15 Failures (And How to Fix Them)](https://www.pewpewtactical.com/ar15-failures/)**. AR-15 Gas Systems ----------------- Now that you're a pro with AR-15 barrels, we'll go into the gas system. There are two major types, Direct Impingement (DI) and Piston. DI is the original design while piston only really became popular within the last few years. ### Direct Impingement vs. Piston An AR-15 works by directing the hot gas behind the bullet into the gas tube (where the triangular front sight block is above), which then uses the gas to either move a piston or send it directly back (direct-impingement). When the force is applied, it makes the bolt unlock, move back, expel the spent casing, and strip a new cartridge into the chamber. The vast majority of ARs are direct-impingement. However, some AR-15s use a piston system where the gas is expelled through the barrel and onto a piston that runs above the barrel. The gas then pushes the piston, which is attached to the bolt carrier by a rod, backwards. Piston-driven ARs do have some benefits. **Pros of Piston:** - Cleaner since dirty gas is vented away - Can be more reliable in bad environments (water, dust, etc) **Cons of Piston:** - Costs more than traditional DI system - Weighs more and puts weight in front (although the gap has closed considerably) - Proprietary parts unique to each manufacturer Unless you're needing to shoot coming out of water or in very dusty environments with limited ability to clean, a direct impingement system will work fine. If that's you...here's our review of the [**Best AR-15 Pistol Uppers**](https://www.pewpewtactical.com/best-ar-15-pistols/). With proper maintenance, a DI AR-15 is a highly reliable weapon, and it is what the US military uses. ### Direct Impingement Gas System Lengths The gas system length refers to the distance to the gas hole, or where the triangular "front sight base" or FSB sits on each barrel above. The rifle-length gas system is traditionally used for 18-inch or longer barrels, but there is the oddball "**[Dissipator](https://www.pewpewtactical.com/what-is-an-ar-15-dissipator/ "What is an AR-15 Dissipator?")**" model which has a rifle length system on a 16-inch barrel. For 16-inch barrels, the primary choice is between carbine and mid-length gas systems. We believe the **mid-length has the advantage** since it allows a longer sight radius if using a front sight base (which doubles as the front sight). It also gives you more possible handguard/rail space since the handguard goes from the upper receiver to the front sight base. It should also theoretically offer a smoother/softer shooting rifle since the increased distance will allow gas pressure to lower before going back into the rifle. But, this is influenced by a lot of other factors including gas port size, buffer, and spring combinations. ### Gas Blocks AR-15s used to come with a front sight block (FSB) but they have fallen out of favor due to free-float handguards becoming affordable. Now, gas blocks are smaller and hidden under the handguard. Some of these gas blocks are also adjustable so you can choose just the right amount of gas to make your rifle/ammo combination run smoothly without unnecessary recoil. If you know you'll want to add stuff to your AR, skip the headache of grinding down the FSB or getting a new upper, and start off with a free-float handguard. ### Handguards Front sight blocks almost always force you to utilize **non-free-floating** handguards. This means that the two-piece handguard touches the barrel at the FSB and can create inconsistent pressure on the barrel, with the potential to negatively affect accuracy. But for non-competitive shooters, you'll be fine with a non-free-floated gun. There are some aluminum variations, but most are polymer, which still work great and are more affordable. Picatinny rails are rectangular-slotted handguards that **were the gold standard** for rails that could attach accessories. Below is an example of a quad rail, so named because it has Picatinny rails on all four sides. These were a big leap over plastic handguards. They offer ultra-strong attachment points, but they weigh quite a bit. While quad rails still remain in use, many manufacturers have gone to slimmer, free-floating handguards and low-profile gas blocks. These allow more consistency in your shots since there's nothing touching the barrel along the handguard. The two most common attachment methods for these newer-style handguards are M-LOK and KeyMod. Both wrestled to become the new industry standard, with M-LOK coming out victorious. Plenty of companies still make KeyMod rails and accessories, but almost no manufacturers are selling complete guns with KeyMod stuff. Everyone has gone over to M-LOK for the most part. If you aren't sure which route to go, check out our article on the [**Best AR-15 Handguards**](https://www.pewpewtactical.com/best-ar-15-handguards/). AR-15 Bolt Carrier Group (BCG) ------------------------------ ### How the BCG Works In the AR-15 platform, the bolt is housed within the bolt carrier. Together, they make up the **bolt carrier group**, **or BCG**. The BCG is essentially the engine that runs the AR-15. When you pull the charging handle back, it pulls the BCG rearward. And when you let go of the charging handle, the BCG moves forward, strips a round out of the magazine, and puts a round into the chamber. When you pull the trigger, the hammer releases and strikes the back of the firing pin, which is also in the BCG. The firing pin hits the cartridge's primer, which sparks the gunpowder and sends the bullet down the barrel. Then the gas system comes into play. ### Semi-Auto vs Full-Auto BCG The BCG comes in semi-auto (SA) and full-auto (FA) varieties. Having a full-auto BCG **does not** instantly make your gun fully automatic. Normally, it is used to slow down the cycling rate due to its increased mass (see the right ends of the BCGs below). Note that they should be overall the same size, but the bottom FA is shrunk in the picture. The full-auto has a shroud to protect the firing pin from hitting the hammer during re-cocking. You can see the right-most BCG has more of the firing pin exposed and less weight on the bottom, making it a semi-auto version. It doesn't really matter which one you get, but most higher-end manufacturers will put in the FA BCG. Most guns are over-gassed so they can run a larger variety of ammunition, so the additional mass and firing pin protection are advantageous. What do I mean by **over-gassed**? In an ideal world, the gas block would send back the bare minimum amount of gas to move the BCG back so it can eject the spent casing and strip another round. However, there's the problem of weaker ammo, which would produce less gas, and also the crud that accumulates and can slow everything down. Therefore, manufacturers usually let a much larger amount of gas cycle through to brute force through the issues. A heavier BCG and shrouded pin minimize the negative effects of a lot of gas. There are also lighter BCGs for competition guns, but don't worry about that for now! But if you really want to, we have you covered **[here](https://www.pewpewtactical.com/best-lightweight-bolt-carrier-groups-bcg/)**. ### BCG Coatings One of the newest trends is to have coated BCGs, which are designed to make them easier to clean, require less lubrication, and last longer between cleanings. Nickel-Boron is pretty standard now, along with Titanium Nitride, Hard Chrome, and Black Nitride. If you're running your gun hard constantly or use a suppressor that deposits more gunk back into your BCG, you might benefit from a coated BCG. Otherwise, you can use a regular BCG. Keep it clean and lubed, and it will run just fine. Check out our [**Best BCGs**](https://www.pewpewtactical.com/best-bolt-carrier-group/) article for our favorite picks. Buffer Tubes, Triggers, & More ---------------------------------- ### Buffer Tube On an AR-15, the buffer tube houses the buffer that cushions the rearward travel of the BCG, as well as provides an attachment point for stocks and braces. There are two standard variants of buffer tubes --- mil-spec and commercial. This issue arises more frequently when building a lower from scratch. Just go with mil-spec instead of commercial, since it is stronger. Also, make sure to match up the carbine/rifle lengths according to your barrel. ### Triggers AR-15 triggers can come in many different flavors, but many guns come with standard mil-spec triggers that typically break around 4.5-6 lbs. Some mil-spec triggers may be a little gritty at first, but they will smooth out with use. Most mil-spec triggers are still pretty decent in the general scheme of things and are usually quite reliable. However, I will say that the first upgrade I do to many of my rifles is upgrading the trigger. There are a ton of aftermarket triggers out there that can give you a significantly cleaner, lighter, and more repeatable trigger pull. Better triggers can help take out a lot of shooter error and help you achieve better accuracy and faster shooting. Triggers are a very common and popular part to upgrade for those reasons. Check out our list of the **[Best AR-15 Triggers](https://www.pewpewtactical.com/best-drop-in-ar-15-triggers/)** to see some of our favorites. ### Optics Iron sights, while useful, are somewhat outdated on modern AR-15s. Many AR-15s don't even come with any sort of sights on them at all these days. This is because they expect the user to select an optic of their choosing and install it on their gun. Optics can help you make extremely precise shots or help you acquire your target quicker, and sometimes both at the same time. A solid optic will help you wring the most performance out of your rifle. There are various types of optics depending on your intended application and your budget. There are so many that we have dedicated an entire article to the **[Best AR-15 Optics & Scopes](https://www.pewpewtactical.com/best-optics-scopes-ar-15/)**. Why the AR-15? -------------- The AR-15 is one of the most popular sporting rifles for many reasons. Here's what we think are the top reasons: ### Used by the US Military Or rather, it looks like the M4s and M16s used by the US military. The civilian AR-15 is a semi-automatic rifle compared to the automatic or select-fire capabilities of some military versions. That means one press of the trigger is a single PEW! instead of a fully automatic, where a single press of the trigger will be a continuous stream of PEW! PEW! PEW! until you let go. ### Shootability The AR-15 is very light recoiling when chambered in its native 5.56x45mm round. It is also very ergonomic when **[compared](https://www.pewpewtactical.com/best-semi-automatic-rifle-for-beginners/)** to other popular rifles such as the AK-47. Check me out dumping 10 rounds on a PSA... There are also **tons of aftermarket products** to customize the look, feel, and performance of your AR-15 to match your personality and intended use. However, there's a prevailing stigma (especially online) of putting too much possibly unnecessary stuff on your gun, and making you a "mall ninja" or being too "tacticool." In my opinion, it's your gun and your money, so do what you want to do as long as it's still safe and legal! I am also very obsessed with upgrades, different uppers, and all the cool stuff you can do with them, as you can see from a selection of my upgraded uppers below. There are so many modifications that it would be way too much to talk about them here, but if you want to know more, just slide on over to our write-up on the [**Best AR-15 Upgrades**](https://www.pewpewtactical.com/best-ar-15-upgrades/). Oh, and modularity means they are easy to clean and maintain. Check out how simple it is in our video below! [Embedded content](https://www.youtube.com/embed/MFiAgeRdFbg) AR-15: To Build or To Buy? -------------------------- For a first AR-15, we would **heavily suggest** buying a complete rifle from a reputable manufacturer. Or at the very least, get a complete upper ([**Guide to Best Uppers**](https://www.pewpewtactical.com/best-ar-15-complete-upper-receiver/)) and build your own lower. It's like Legos for big kids, and you can see our recs at [**Best AR-15 Lowers**](https://www.pewpewtactical.com/best-ar-15-lower-receiver-for-any-budget/). Now there are also options to simply get a complete upper AND a complete lower. This way saves some money too, since companies are charged an 11% tax on complete firearms. Buying complete receivers eliminates most potential headaches by having a solid warranty behind your parts. Additionally, if you're just starting out, you probably don't know what you like or want, so I recommend going with a standard configuration. A factory gun doesn't cost that much more than building your own (and is sometimes less). Factory guns also have a much better resale value if you ever decide to part ways (or upgrade). However, there's something **very satisfying** about building your own "Franken-gun" build, but we recommend doing it for your second or third AR-15. However, it's not terribly difficult, and you'll get to choose every component in your gun. If you're curious, check out our **[How to Build an AR-15 Lower Receiver](https://www.pewpewtactical.com/build-ar-15-lower-receiver/)** and **[Upper Receiver](https://www.pewpewtactical.com/build-ar-15-upper-receiver/)** articles just to see your future. Next, we delve into the various components of the AR-15 before assembling them and recommending specific combinations based on your intended use.