AR-15 Courtesy of Straight 8 Photography

AR-15 Basics – Part 1

Beginner’s Guide to the AR-15 Platform

Part 1: A Brief History, it’s Nomenclature and Figuring out What’s Best for you

We’ve talked about handguns and long range rifles in previous articles and in this one we’ll talk about the AR-15; arguably the most popular sporting rifle in the country. Inevitably when you start talking about the AR-15 an argument will start about which is a better rifle the AR-15 or the AK-47/74. While both have their good and bad points, I prefer the AR series for a couple of reasons. The M16/M4 is the standard battle rifle for the US Armed Forces and federal, state and local law enforcement. In most cases parts for the M16/M4 are interchangeable with the AR-15. This means that during an apocalyptic event like a zombie outbreak, parts, magazines and ammunition could be found just about anywhere.

There are some important differences between the military and civilian versions of the rifle and we’ll talk about those as well as the differences in the AR family of weapons.

Before we get into the AR-15, I feel it’s important to know a little bit about the M16/M4 and its history.

M16 AR-15 Basics zombie education alliance


We’ve all seen the M-16 on television or in the movies. For years most people assume that the AR-15 is the semi-automatic version of the M-16, and while that is true the original designation was AR-15. In 1959, ArmaLite sold its rights to the AR-15 to Colt. The AR designation was kept but contrary to popular belief, AR does not stand for Assault Rifle, it meant and still means ArmaLite Rifle. The AR-15 was first adopted in 1962, by the United States Air Force for their security police; ultimately the AR-15 received the designation M16. The U.S. Army began to field the XM16E1 in 1965, with the majority of them going to the Republic of Vietnam Army and the newly formed Airmobile Divisions, chiefly the 1st Air Cavalry Division.

There were a lot of problems with first M16s, mostly failures to eject spent rounds that caused the weapon to jam. I have a good friend who was in the 1st Air Cav in Vietnam and he has told me that when he was first issued an M-16, it jammed constantly and usually at the absolute worst time.

The problem was traced to the powder used in the round and this, as well as some other reliability problems, were fixed and the M-16A1 was born. The A1 was followed by the A2 and it was that model that was later developed into the M4; the current issued rifle is the M4 carbine. The M4 carbine has a three-round burst firing mode, while the M4A1 carbine has a fully automatic firing mode. Colt’s Manufacturing LLC retained the AR-15 designation and offered it for sale to civilian and law enforcement markets as a semi-automatic version of the M-16.

Semi-automatic AR-15s for sale to civilians are internally different from the full automatic M16. Outwardly they appear nearly identical with the upper receiver being slightly different with an area milled out for clearance. Internally, however, they are quite different being the hammer and trigger are of a different design. The bolt carrier and internal lower receiver of semi-automatic versions are milled differently, so that the firing mechanisms are not interchangeable. This was to satisfy ATF requirements so civilian weapons cannot be easily converted to full-automatic. In the late 1970s and early 1980s, modifications to the AR-15 were rampant with “Drop In Auto Sear” or “lightning-link,” conversions to full automatic. However, these very straightforward modifications, unless using registered and transferable parts made prior to May 19, 1986, are now illegal.

In 1986, The Firearm Owners Protection Act (Public Law 99-308) redefined a machine gun to include individual components that can be used to convert a semi-automatic firearm to full-automatic. I can not stress this enough. If you convert an AR-15 to full automatic and you get caught, you are going to prison. The ATF’s Firearms Enforcement Division takes a very dim view of this.


Having said that, there are parts that are interchangeable and having them installed on your AR-15 does not make it a machine gun under federal law. The full auto bolt and bolt carrier are interchangeable, and having one in your rifle is legal. One thing to note on the interchangeability of parts, the AR-15 comes in two flavors: Commercial and Mil-Spec. Over the course of this article, I’ll point out the differences between AR-15 and M16/M4 parts and Mil-Spec vs Commercial. But regardless, the various types function the same way.


The AR-15 consists of three major components: the upper receiver, the bolt and bolt carrier, and the lower receiver.

AR_parts AR-15 Basics zombie education alliance

The upper and lower receiver are held together by two pins, pushing the rear most pin out will allow the upper receiver to tilt forward for field cleaning, pushing both pins out will allow the upper and lower assemblies to be separated.


There are two basic operating systems for the AR-15, Direct Impingement and Gas Piston with the most common being Direct Impingement. All of the ones I own are Direct Impingement.

Direct impingement is the original technology used in the M16 and its variants devised by Eugene Stoner in the 50’s (see video below). Propellant gas from the fired round is bled through a small hole located in the barrel, this gas is then channeled through a very small tube where it is fed to directly contact (or impinge) the bolt carrier. At this point the gas is pushed to the rear of the rifle forcing the bolt carrier back the spent case is extracted and ejected. As the bolt carrier moves rearward it pushes the buffer back compressing a spring It is then pushed forward by spring-loaded action, and strips an unspent round from the magazine, loading it directly into the chamber of the barrel. I purposely fired hundreds of rounds though one of my ARs without cleaning it to see if I could induce and malfunction and never could.

A gas piston is the same operation as used by the AK-47 (see video below). While it is more or less the same as direct impingement systems, there are a few differences. Firing a round again feeds propellant gases into the barrel. But instead of being forced into a tube as it is in a direct impingement system, it is contained in a separate cylinder. This cylinder holds a piston, as the gas moves the piston, it in turn pushes the bolt carrier rearward to extract the spent rounds and feed in the new round, the bolt carrier is pushed forward to the closed position by a spring just as with direct impingement.

Which is better is another question that is sure to start an argument. My only comment is that the military uses direct impingement and the vast majority of the civilian AR-15 are direct impingement.

Let’s examine a direct impingement AR-15 in detail.

Starting at the front and working our way back, the example shown below is a stripped AR-15 that uses a 300 AAC Blackout, 7.62×35mm, cartridge. Other than the caliber the component parts and operation are the same as a standard AR-15 in .223/5.56.

AR-15 in .300 AAC Blackout

AR-15 in .300 AAC Blackout


AR-15 with fixed stock

Fixed Stock .308 AR-10

Starting with the barrel, the end — unless the barrel is target crowned as the .308 AR-10 shown above — will be threaded 1/2×28, which is standard for 5.56/.223. It will vary with other caliber. The one shown is for a 300 Blackout and is threaded 5/8×24. This particular barrel is stainless steel.

muzzle brake ar-15 basics zombie education alliance

The flash suppressor shown above is Red Jacket Firearms shorty muzzle brake and is also threaded for a suppressor. Next is the gas block and gas tube, under the gas block there are holes in the barrel for the gas to escape, some gas blocks are adjustable, these adjustments allow you to control the amount of gas and in turns controls cycling and operation, the one shown is not adjustable.

gas tube ar-15 basics zombie education alliance

The gas tube and barrel is covered by the forward hand guard. There are a huge number of options for the hand guard and it is really up to you.

Next is the upper receiver. There are a number of options but they are all more or less the same, the barrel is attached to the upper receiver, it contains the ejection port, in some cases the forward assist, charging handle and the bolt and bolt carrier. Some upper receivers have an integral carry handle and some have rails.

A3 upper receiver ar-15 basics zombie education alliance

Upper Receiver

Next is the bolt, bolt carrier and charging handle. This is an example of the difference between the full auto bolt carrier and the civilian version. As shown  below there is a notable difference between the two.

AR-15 Basics zombie education alliance

The charging handle is used to move the bolt carrier rearward to charge the weapon. There are number of options for the charging handle. Shown is a standard one alongside one with an extended ambidextrous latch.

ar-15 basics charging handle zombie education alliance

The upper receiver is not considered a firearm and therefore is not controlled. You can order one and have it shipped directly to your home, unless the barrel is less than 16 inches. Anything under 16 inches is considered a short barreled rifle when mounted on a lower receiver, and is controlled by the National Fire Arms Act (NFA) and must be registered with all of the associated paperwork and tax stamp.

Upper receivers are functionally all pretty much the same, they can be made out of metal like aluminum either cast or billet, titanium, or a composite like carbon fiber. They can be purchased as a stripped unit, assembled, or as a complete upper as mentioned above.

Short Barrel AR ar-15 basics zombie education alliance

AR Short Barrel – Bushmaster CM-15

Simply having a short barreled upper is not in itself illegal. However, if you have a short barreled upper and a standard 16 inch barreled AR-15 and you are caught with both you are going to jail, because the standard AR-15 can easily be converted to a Short Barreled Rifle that you have not registered.

In part two of this series, I’ll be discussing the lower receiver and the AR’s ammunition.

Special thanks to Straight 8 Photography for the featured header image.

About Mike

Mike was an associate editor for Zombie Training before co-founding ZEA with former managing editor TS Alan. Mike has been in the aviation industry for over 30 years first in the military and then with an airline and manufactures, an avid racer and is a licensed ASA, NHRA, IHRA driver, Mike has taught performance driving and drag racing and has raced all over the world. He is a NRA Range Safety Officer and is working on an NRA instructors certificate. While writing for Zombie training Mike formed relationships with several manufactures and training groups that will help the ZEA bring the latest products and training to you Mike currently resides in the desert southwest.