AR 15 Lower Receiver And Ammunition | Basics Of The AR-15 Part 2

Featured | AR15 with .223 ammunition | AR 15 Lower Receiver And Ammunition | Basics Of The AR-15 Part 2

Get down to the nitty-gritty of the AR 15 lower receiver and ammunition as you learn more of the AR-15 basics here.

RELATED: AR-15 Barrel Install | Tips & Tricks

In this article:

  1. The AR 15 Lower Receiver
  2. Ammunition
  3. The AR-15 Pistol

Intro to the AR 15 Lower Receiver and Ammo

The AR 15 Lower Receiver

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 which you can use 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 which allows full-auto or semi-auto. Like the upper, different materials make the lower receiver, the most common being aluminum or composite.

Purchase Limitations


You can purchase the lower receiver as a bare unit or complete. But, unlike the upper, it has a serial number and therefore controlled by the Bureau of Alcohol, Tobacco, Firearms, and Explosives Firearms Enforcement Division.

This means you cannot buy one and have it shipped to your house. You have to ship it to someone who is in the firearms business with a Federal Firearms License, then have it transferred to your ownership only after an FBI NICS background check.



If you are good with gun mods, there is an alternative. You can purchase what they call an 80% lower.

This is an 80% finished lower receiver which doesn't have a serial, and because of this, it is not considered a “firearm” by the ATF. The only requirement is to drill a couple of holes and mill out a section and you have a functioning lower.

The one on the picture below is made from billet aluminum. However, if you decide to go this route you need to be careful if you decide to sell it.

Parts Breakdown of the AR 15 Lower Receiver


Functionally, the fire control group are the trigger, sear, hammer, and spring. A couple of pins hold them together in the lower.

You can purchase individual parts, as a kit or a drop-in trigger. The bolt and magazine release are also in the lower receiver and are also held in by pins.


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 the felt recoil.

As mentioned before, the buffer tube can be commercial or mil-spec, and the stock fits over the buffer tube.


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


The buffer tube connects 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.

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 one of those things you need to make sure — all the parts will work with whatever caliber you intend to shoot.

Part of the lower is the magazine as 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.


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 you know about the lower receiver, let’s discuss ammunition and the various calibers that can be used.

RELATED: Purchasing An AR-15 | A Buyer’s Guide To Not Getting Ripped Off


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 .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 have a different assembly.

And, there is enough of a difference since 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.

As mentioned in the first part of this series, the most common caliber of ammunition for the AR-15 is .223/5.56, but it 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.

Ammo Calibers


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

  • 17 Remington
  • .17/223
  • .20 Tactical
  • .20 Vardag
  • .204 Ruger
  • .20 Practical
  • .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, you can purchase these uppers to turn your lower into a crossbow, shotgun or a bolt action rifle.

The Latest on AR-15 Ammunition

Recently, AR-15 ammunition has been in the news because of a proposal to ban of 5.56×45 SS109/M855 ammunition. Our friends in Washington took it upon themselves because it could be fired from an AR pistol and is “armor-piercing.”

While the M855/SS109 does have a steel penetrator tip, this does not make it “armor-piercing.” The design of the M855 round was to increase penetration at longer ranges.

According to Title 18 of the United States Code 18 USC sec. 921(a)(17). which says;

(17)(A) The term “ammunition” means ammunition or cartridge cases, primers, bullets, or propellent powder designed for use in any firearm.

(B) The term “armor piercing ammunition” means—

(i) a projectile or projectile core which may be used in a handgun and which is constructed entirely (excluding the presence of traces of other substances) from one or a combination of tungsten alloys, steel, iron, brass, bronze, beryllium copper, or depleted uranium; or

(ii) a full jacketed projectile larger than .22 caliber designed and intended for use in a handgun and whose jacket has a weight of more than 25 percent of the total weight of the projectile.

(C) The term “armor piercing ammunition” does not include shotgun shot required by Federal or State environmental or game regulations for hunting purposes, a frangible projectile designed for target shooting, a projectile which the Attorney General finds is primarily intended to be used for sporting purposes, or any other projectile or projectile core which the Attorney General finds is intended to be used for industrial purposes, including a charge used in an oil and gas well-perforating device.

This is typical federal government babble but the important part is in section (i). It says the projectile core should only be entirely made up of one of the listed materials; the SS109/M855 has a core made of lead and steel.

The ATF specifically states the SS109/M855 bullets are exempted.

If you choose to outfit your lower with some odd caliber, keep in mind in an emergency, 6.5mm ammunition is probably going to be hard to find so you should have a standard caliber upper as a backup.

The AR-15 Pistol


Now let’s briefly discuss the AR-15 pistol. This version of the AR is a standard lower with a pistol length buffer and buffer tube which won’t accept a stock.

The upper can have a barrel 7.5 to 11 inches in length. As mentioned before, an AR-15 with a barrel length of fewer than 16 inches with a stock or is intended to be fired from the shoulder is a Short Barreled Rifle.

In the next part of this series, we will examine shooting the AR as well as some basic tools you will need to maintain your AR-15.

Watch this video from Lucky Gunner Ammo on how to use an AR-15 if you're a first time owner of an AR-15: 

The AR 15 is both the most hated and loved rifle in America. For gun owners, this is a must-have in your collection. So before you try to own one or are a proud owner now, make sure you know your AR 15 inside and out, and the responsibilities which come to owning one.

Have you made a recent gun mod to your AR 15? How did it go? Let us know your thoughts about it in the comments section below!

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AR 15 Lower Receiver And Ammunition | Basics Of The AR-15 Part 2 |

Editor’s Note: This post was originally published on May 18, 2015, and has been updated for quality and relevancy.

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