In this guide to AR-15 basics, we will explore its operation, explain the names of the parts, and review the pros and cons of purchasing a complete rifle versus building your own.
We will also highlight some areas to pay attention to regardless of if you are buying or building your own.
We have over 20 years of experience in the military and building our own rifles. My engineering background in machine design helps me understand the technical details of how the AR-15 is designed and functions.
Keep reading to learn everything you need to know about AR-15s!
Table of Contents
AR-15 Basics
The AR-15 is one of the most popular and most common firearms in the US today. It is easy to use, modular, and an open platform that any manufacturer can produce. It has been thoroughly tested and is very reliable as well.
What is an AR-15?
An AR-15 is a civilian version of the military’s M-16 and M-4 rifle. It was originally built by ArmaLite, which gave it the “AR” name. It stands for “ArmaLite Rifle” and not “Assault Rifle” like politicians and the media like to portray.
Since the AR-15 is built for civilian use, it is not used by the military. However, some of the individual parts are designed similarly and are interchangeable. When you see “Mil-spec,” the part was built to military specifications.
The main difference between the military M-4 and the civilian AR-15 is that the M-4 is capable of fully automatic fire. Civilians are not allowed to possess automatic weapons without special government licenses. The AR-15 is a semi-automatic firearm that only fires one bullet each time the trigger is pulled.
What Caliber is an AR-15?
The most common caliber for the AR-15 is the .223 Winchester or 5.56x45mm (or 5.56 NATO). These are essentially the same cartridge. However, the 5.56 is slightly more powerful (see our bullet sizes guide for more).
Since the AR-15 is a modular and open design, manufacturers have produced them in calibers from .22 to .50. I even have a BB gun that was designed from many of the same AR-15 parts.
Some of the more common calibers are:
- .300 Blackout (great for home defense)
- 6.5 Gredel
- .450 Bushmaster
- 7.62×39
- .458 SOCOM
Some AR-15s are designed to shoot pistol ammunition like 9mm and .45ACP. These are called Pistol Caliber Carbines (PCCs). They look just like rifles but use cheaper pistol ammo. This makes them great for personal defense and training, since the cost of ammo is so much less.
To save as much as possible on ammo, be sure to check our top picks for the Best Places to Buy Ammo Online.
Basic Operation of the AR-15
The operation of the AR-15 is a simple and reliable design.
A standard mil-spec AR-15 is a gas-operated firearm. This means that the high-pressure gas that propels the bullet down the barrel is also used to operate the action.
When the trigger is pulled, the BCG (Bolt Carrier Group) firing pin contacts the ammo cartridge’s primer. This ignites the gunpowder inside the cartridge. The burning powder creates a high-pressure gas that pushes the bullet down the barrel.
AR-15 barrels have a small hole that allows the gas to pass back to the upper receiver. At the hole in the barrel, a “gas block” is installed that directs the gas into a “gas tube.” The gas tube ends inside the upper receiver at the BCG.
The gas then pushes the BCG to the rear of the receiver, where it contacts the buffer. The buffer compresses the buffer spring, pushing back against the buffer and returning the BCG to the forward position.
As the BCG moves backward, the extractor built into the BCG pulls the spent cartridge casing out of the chamber and ejects it from the upper receiver. As the BCG returns forward, it pushes the next round from the magazine into the chamber.
Then, the cycle repeats.
Here is a great animation of how an AR-15 works
Understanding AR-15 Gas Systems
There are four different gas system lengths available for an AR-15. Pistol, Carbine, Mid-length, and Rifle.
The only difference between these four systems is the distance between the gas hole in the barrel and the receiver. As this changes, so does the length of the gas tube.
Pistol gas systems are the shortest, with the gas tube being 6.75 inches long.
Carbine gas systems have a gas tube that is 9.75 inches long.
Mid-length gas systems have a gas tube that is 11.75 inches long.
Finally, Rifle gas systems have a gas tube that is 15 inches long.
In general, shorter gas systems will make the AR-15 cycle faster and harder. This is because the gas has less distance to travel to the receiver.
As a general rule, always select the longest system that your selected barrel length will allow (very short barrels, for example, will only come in pistol length). However, if you want a rifle that is very reliable in adverse conditions, go with the carbine length.
If you want control over the amount of gas that is returned to the receiver, adjustable gas blocks can be installed. These have a small screw on them that vary the size of the hole through the gas block.
Typically, these are unnecessary unless you want to tune an AR-15 to specific ammunition, conditions, or muzzle devices. If you plan to use a suppressor, one may be needed since a suppressor can change how fast the gas escapes from the end of the barrel.
For more on gas systems, see this guide from Primary Arms.
Parts of an AR-15
As I mentioned, the AR-15 is a modular firearm designed to be disassembled easily. It can be broken down for maintenance very quickly without any tools.
There are two main parts of an AR-15: the upper receiver and the lower receiver, which are commonly referred to as “upper” and “lower.”
AR-15 Lower
The lower receiver of an AR-15 is the heart of the platform and the part that the government considers a “firearm.” It can only be purchased through a transfer with an FFL (Federal Firearms License) dealer and cannot be delivered directly to your door. However, it can be purchased online. See our guide on how to buy a gun online for more.
Lower receivers are typically made out of forged Aluminum and contain the trigger, safety, bolt catch, magazine release, buffer, and stock.
You can purchase a lower receiver in three different ways.
The first way is a “stripped” lower. This is just the aluminum receiver with no additional parts. This is the starting point if you want to build your AR-15. Even though it is just a piece of metal, it contains the serial number and is still considered a firearm by the government.
To buy a stripped lower receiver, you still have to complete the same background check paperwork through an FFL dealer, just like you would for a full firearm.
See our guide to the Best AR-15 Stripped Lowers for our favorites!
The second way is a “complete” lower. This fully assembled lower receiver has all the parts needed for a functioning firearm. Again, the government still considers it a firearm even though it is not capable of firing by itself.
The third way, of course, is to buy a complete rifle.
Lower Parts Kit
If you do decide to build your own AR-15, a lower parts kit contains all of the parts needed to assemble a stripped lower into a complete lower. I also like to keep one on hand for spare parts.
The lower parts kit can be ordered online and delivered directly to you since it is not considered a firearm.
The parts needed to build an AR-15 lower are:
Takedown pins: These pins hold the upper receiver to the lower receiver.
Bolt Catch: This device holds the bolt of the AR-15 open either manually or after the last round is fired from a magazine.
Safety Selector: This is the switch that turns the safety on and off.
Magazine Release: This is the device that the user presses to drop the magazine out of the rifle.
Trigger: Self-explanatory, you pull this, and it goes bang. However, there are many different trigger options. A “Mil-spec” trigger is the basic level with most parts kits. There are many different triggers than can cost as much as $300.
See our review and guide on the Best AR-15 Triggers for our favorites.
Trigger Guard: This small piece goes around the trigger to guard it from being hit from below.
Grip: This is the grip where the AR-15 is held at the trigger. It is one of the most critical pieces from an ergonomics perspective. If the grip is uncomfortable, you just won’t shoot well. See our Best AR-15 Grip Review for our favorites.
Stock: This is the butt of the rifle that is held against the shoulder/chest when firing. It is the second most critical piece ergonomically. It needs to fit the shooter well. For AR-15 pistols, this becomes the arm brace (see below). See our Favorite AR-15 Stocks for our top picks.
Buffer Tube Assembly: This is the tube on the back of the AR-15 that the stock is mounted to. It contains the buffer and buffer spring, which are critical for the AR-15’s proper operation. The buffer tube is held onto the lower receiver by the castle nut and end plate.
There are a few different ways to buy lower parts kits.
- Full lower build kit: includes everything that is needed in the list above.
- Lower parts kit: Typically includes everything but the stock and buffer tube assembly. Some may or may not include a grip.
- Lower parts kit minus FCG: These are the same as a regular parts kit but they do not include a trigger (FCG=Fire Control Group).
Below is a full build kit. The six bags on the right make up a lower parts kit minus the FCG.
See our AR-15 Lower Parts Kit Guide for more and our favorites!
AR-15 Upper
The upper receiver is the top section of the AR-15, which includes the barrel. It is held onto the lower receiver by the two takedown pins in the lower. Like the lower, it is typically made from forged Aluminum.
The government does not consider the upper receiver a firearm, so it does not have to go through an FFL transfer. You can buy these online and have them shipped directly to your home.
Similarly to the lower receiver, an upper receiver can be purchased in three ways. Stripped, complete, or with a full firearm.
A stripped upper receiver is just an aluminum receiver with no other parts included.
A complete upper receiver includes the receiver, barrel, muzzle device, handguard, gas block, gas tube, and forward assist. Some will include the BCG and charging handle.
Upper Parts Kit
Buying the parts needed for an upper receiver is not as straightforward as it is for a lower receiver. This is why I typically like to buy a complete upper when possible.
The main difference in upper receivers is the handguard. There are two basic types of handguards: standard military style and free float. The standard military handguards attach to the barrel and look similar to the M-16.
Free-float handguards attach to the upper receiver, not the barrel. Rifles with free-float handguards are inherently more accurate, and they allow accessories to be mounted. Personally, I think they look better and are more ergonomic.
Free-float handguards typically attach to the barrel nut. This is where it gets tricky since the upper receiver needs to match the handguard. Some handguards are not compatible with some upper receivers.
Here are the parts needed to build an upper AR-15 receiver.
Handguard: Many different designs, lengths, and attachment methods are available.
Bolt Carrier Group (BCG): This is essentially the AR-15’s “action.” It contains the firing pin and moves back and forth over the magazine, extracting spent brass from the chamber and feeding a new round back into the chamber after a round has been fired.
Charging Handle: The shooter uses this to manually move the BCG to load a round from the magazine into the chamber. These can range from simple Mil-spec designs to ambidextrous charging handles of various sizes. See our Favorite AR-15 Charging Handles for our top picks.
Forward Assist: This is a military carryover and really isn’t needed. A mil-spec upper receiver will have a forward assist, while some upper receivers are available with it removed. It is meant to force the BCG into battery if it is very dirty or full of debris.
Barrel: The barrel inserts into the upper receiver and is held in place by the barrel nut. Various profiles, calibers, lengths, gas system lengths, and coatings are available.
Muzzle Device: This is simply the device that screws onto the end of the barrel. It can be anything from a standard military-style flash hider, recoil compensator, or even a full suppressor.
Gas Block: The gas block attaches to the barrel and directs gas from the barrel to the gas tube. As mentioned, adjustable gas blocks are also available, but they are typically only for the advanced shooter or when using a suppressor.
Gas Tube: This tube attaches to the gas block and enters the upper receiver. It delivers gas from the gas block back to the BCG in the upper receiver. The gas tube length must match the barrel!
Building vs Buying an AR-15
I see it often suggested that building an AR-15 is cheaper than buying one. I find that for higher-end brands, this may be true, but in general, it is not true. People forget to factor in the tools and time it takes to build a rifle, not to mention the time spent researching and ordering components. The cost of an AR-15 can range from less than $500 for a budget rifle, all they way up to $2000 or $3000.
If you like working with your hands and are mechanically minded, building your own rifle is a lot of fun. But more importantly, it requires you to learn how the rifle works. In turn, this is especially valuable for preparedness and survival situations as you can troubleshoot and repair any AR-15 yourself.
I find the best middle-of-the-road approach is to build the lower myself and then buy a complete upper already assembled. I don’t find an advantage in building the upper unless I build something very specific. Plus, I can usually find an upper on sale that is cheaper than the individual parts I can buy.
See my strategies for saving money in my Budget AR-15 Build Guide. I included a full parts list and build a full rifle for less than $350!
When I buy a complete lower, I usually change out the grip, stock, and maybe the trigger. In the end, it is cheaper to just buy what I want in the beginning instead of upgrading later.
If you have a strict budget, buying a full rifle makes the most sense. Plus, it will come with a warranty, service, and support. Check out our Bear Creek Arsenal AR-15 review for a good example of a budget rifle.
In summary, here are the pros and cons of building your own AR-15.
Pros:
- Learn functionality and how to troubleshoot/repair yourself
- Can select individual components instead of spending money on upgrades later
- Can build to a specific purpose/intent
- Easy to make changes later
Cons:
- Need tools and time to learn/build
- Need to research what components to use
- Usually more expensive than buying a complete rifle
- No warranty
- No technical support from factory
AR-15 Rifle vs AR-15 Pistol
There are some critical details to remember when building your own rifle or buying an upper.
Keep in mind we are not lawyers and do not give legal advice.
Federal law in the US restricts rifle barrel lengths to a minimum of 16 inches. Any barrel length shorter than this becomes an SBR (short barrel rifle), which has additional restrictions that require a tax stamp and other requirements set by the National Firearms Act (NFA).
So, if you take an AR-15 lower with a stock and put an upper on it with a barrel less than 16” long, you are breaking federal law.
The key definition of a rifle here is the stock. It is designed to be held against the shoulder when fired.
AR-15 pistols are essentially the same as AR-15 rifles, except the stock has been replaced with an “arm brace.” If the lower has an arm brace, any length of the barrel can be used since it is no longer classified as a rifle.
Since many pistol arm braces look similar to stocks, it can be instinctive to shoulder them just like a rifle. A few years ago, there was a discussion that it was illegal to shoulder fire an AR-15 pistol. Since the law only describes what rifles are and not how they are used, this was proven incorrect.
Again, we are not giving legal advice here, so be sure to inform yourself before building your own rifle or pistol. If your lower has a stock, make sure the barrel is over 16” unless you plan to get an SBR stamp.
Learn More About AR-15s
We plan to publish many more guides on AR-15s, including the best upgrades, setups, optics, and more. Be sure to sign up for our newsletter to be notified when these are live.
Also, check back here for a list of new articles below:
- Bear Creek Arsenal AR-15 Review
- Primary Arms SLx 1-6×24 Scope Review
- Budget AR-15 Build Guide and Parts List
- Best AR-15 Flashlight
- Best Home Defense AR-15 Setup
Check out our Self-Defense Guides and Gear Reviews page for a lot more.
Also check out our review of TacPack, a cool tactical gear subscription box that includes a lot of different AR-15 parts each month!
