Stephanie Hayden AR-15 basics - part 2 zombie education alliance

AR-15 Basics – Part 2

Beginner’s Guide to the AR-15 Platform

Part 2: The Lower Receiver and Ammunition

In the first part of this series we looked at how an AR-15 works and touched on some of its nomenclature, including the upper receiver. In this part we will look at the lower receiver along with the ammunition that can use utilized with this carbine.

Functionally all lower receivers work the same. It has the fire control group, which is the trigger, a safety, a magazine eject and a bolt release. In the case of an M16, the safety is also the fire select lever that allows full auto or semi-auto. Like the upper it can be made of many different materials, the most common being aluminum or composite.


The lower receiver can be purchased as a bare unit or complete, but unlike the upper it is serialized and therefore controlled by the The ATF’s Firearms Enforcement Division. This means you can not buy one and have it shipped to your house, it would have to be shipped to someone who is in the firearms business that has a Federal Firearms License, and then transferred to your ownership only after a FBI NICS background check.

If you are mechanically inclined there is an alternative, you can purchase what is called an 80% lower. This is an 80% finished lower receiver and is not serialized, and because of this it is not considered a “firearm” by the ATF. All that is required is to drill a couple of holes and mill out a section and you have a functioning lower. The one pictured below is made of billet aluminum. However, if you decide to go this route you need to be careful if you decide to sell it.


Functionally the fire control group is made up of the trigger, sear, hammer and spring, and is held together in the lower by a couple of pins. It can be purchased as individual parts, as a kit or a drop in trigger. The bolt and magazine release are also in the lower receiver and are held in by pins.

Lower Kit

Lower Kit

The buffer, recoil spring and buffer tube are also part of the lower. The buffer and spring come in many varieties, and control the cycle time of the AR-15 and felt recoil. As mentioned before the buffer tube can be commercial or mil-spec, and the stock fits over the buffer tube.

AR-15 basics - part 2 zombie education alliance

When purchasing an after market stock make sure you know which one you have as they are different sizes, and a fixed stock uses a different buffer tube than a collapsible stock.

AR-15 with fixed stock

AR with a fixed stock

AR-15 basics - part 2 zombie education alliance

M4 with a collapsible stock

AR-15 basics - part 2 zombie education alliance

The buffer tube is attached to the lower by a large castle nut and takes a special tool to tighten. The buffer and recoil spring are held in by a small spring loaded pin.

AR-15 basics - part 2 zombie education alliance

The lower receiver should be marked with the caliber or in some cases it will be marked as multi. For example a 5.56 lower will work with .300 AAC but not .308. This is another one of those things that you need to make sure that all the parts will work with whatever caliber you intend to shoot.

AR-15 basics - part 2 zombie education alliance

100 Round Drum Magazine

Part of the lower is the magazine well. AR-15 magazines can be metal, plastic or composite. I’ve found that some magazines won’t work in one lower but run fine in another. Magazines can hold anything from 5 to 100 rounds with the most common being 30 rounds.

AR-15 basics - part 2 zombie education alliance

These are illegal in California.
Photo courtesy

When purchasing magazines make sure you know the laws of the state you live in. In California a 20 round  magazine will get you thrown in jail, while in Arizona a 100 round is completely legal. Now that you know about the lower receiver, let’s discuss ammunition and the various calibers that can be used.

The AR-15 comes in as many different calibers as there are accessories for it, everything from .22 LR to .50 BMG. The most common caliber is the .223/5.56 x 45. There is a common misconception that .223 and 5.56 are the same. While they are very similar, the 5.56 is assembled differently and there is enough of a difference that a rifle chambered for .223 cannot safely chamber and fire a 5.56 round. However, a rifle chambered in 5.56 can fire a .223 round.

AR-15 basics - part 2 zombie education alliance

As mentioned in the first part of this series the most common caliber of ammunition for the AR-15 is .223/5.56, but that doesn’t mean it’s the only caliber available. One of the reasons the AR is so popular is the variety of options available. In most cases the lower receiver doesn’t change and by simply mounting a different upper you can change caliber, barrel and optics. The options are only limited by your imagination.

As an example, the list below are just some of the calibers that can be fired by a standard mil-spec lower with just an upper change. In most cases it’s the same magazine, bolt and bolt carrier.

  • 17 Remington
  • .17/223
  • .20 Tactical
  • .20 Practical
  • .20 Vartag
  • .204 Ruger
  • .221 Fireball
  • .222 Remington
  • .222 Remington Magnum
  • .223 Remington
  • .223 Remington Ackley Improved
  • 6x45mm
  • 6mm TCU
  • 6x47mm
  • 6mm Whisper
  • .25x45mm
  • 6.5mm Whisper
  • 7mm Whisper
  • 7mm TCU
  • .300 Whisper (.300/221, .300 Fireball)
  • .300 AAC Blackout
  • .338 Whisper
  • 5.56x45mm NATO

There’s probably more but you get the idea, a standard mil-spec lower with just an upper change gives you more flexibility when ammunition may be hard to find. In addition, these uppers can be purchased to turn your lower into a cross bow, shotgun or a bolt action rifle.

If you choose to outfit your lower with some odd caliber, keep in mind that during the zombie apocalypse or other doomsday scenario, 6.5mm ammunition is probably going to be hard to find so you should have a standard caliber upper as a back up.

AR-15 basics - part 2 zombie education alliance

AR-15 basics - part 2 zombie education alliance

AR-15 pistol converted from a Mega Arms AR-15

Now lets briefly discuss the AR-15 pistol. This version of the AR is a standard lower with a pistol length buffer and buffer tube that won’t accept a stock. The upper can have a barrel 7.5 to 11 inches in length. Here is the catch, our “friends” at the ATF have determined that the lower receiver has to be registered as a pistol. What this means is that if you have a rifle lower and you put a pistol buffer tube on it with a 7.5 inch upper, you now have a short barreled rifle not a pistol.

In the next part of this series we will examine shooting the AR as well as some basic tools that you will need to maintain your AR-15. If you missed Part 1, you can find it here.

Our feature image is our friend Stephanie Hayden. Thanks for the photo!

AR-15: Beginner’s Guide to the AR-15 Platform – Part 2 by Mike Zombie Education Alliance.

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.