As soon as you make the decision to put together your own AR platform rifle, you will be faced with options. As we discussed in the last episode of this debate, I suggest you start with the caliber. Knowing what you want to shoot will point you in the direction of AR-15 lower receivers, and that’s where we’ll start today.
AR-15 Lower Receivers: Modularity
The AR-15 was designed to be taken apart, put back together, and combined in numerous ways. The pieces, ideally, fit with similar parts from other manufacturers who work off the same designs. It is, fundamentally, an open-source tool that has (and continues to) evolved over more than half-a-decade.
The AR-15 lower receiver is, at its most basic, simply a shell that holds some key features of the rifle (or pistol, if you’re into that sort of thing). The lower is a shell that houses the trigger group. In front of that, there’s a mag well that presents rounds into the upper via a well-seated magazine.
An AR’s safety sits on the outside of the lower. The bolt release and mag release, too. And the stock attaches to the rear, where a buffer tube fits. The pins that connect the lower to the upper are accessible on the side of the lower.
In short, the AR-15 lower is the control center. The importance of this piece is recognized in its singular inclusion as the serialized component that the ATF considers a “firearm.” The trigger group, controls, barrels, uppers, stocks… everything you might need to make the gun function can be bought online, or in some stores, with no paperwork. The simple shell of the lower, though, requires the same paperwork that you fill out when buying a complete rifle.
The standard for the AR-15. Most are black anodized aluminum of some sort—more on that below. These are everywhere. Prices range from $40 on the low end for a stripped receiver, to… you name it on the high end. $500+ isn’t uncommon.
If you want to shoot 300 blackout from the same lower that you shoot .223 and 5.56, you can find an honest lower that is stamped “multi” in place of the caliber designation common on most AR-15 lower receivers. This designation will go on the paperwork, of course, and on any SBR tax stamps.
Any AK inspired ARs, especially those that take AK mags, will have the front of the mag well chopped at an aggressive angle. This permits the mags to be rocked in. AR mags go in vertically and don’t bend until they’re free from the bottom of the mag well. This distinction typically will allow you to spot a 7.62×39 lower at a glance.
If you are venturing into the larger calibers, you’ll need a larger lower. As the case length increases, the internals of a typical 5.56 lower become quite constrained.
Pistol calibers, on the other hand, are easily used in a standard AR-15 lower receiver. The only concern there is the need for a magazine well conversion kit. There are now kits for the AR that will allow for just about any imaginable caliber.
Some of these, like the .22, can be accommodated by a magazine. But if you want Glock mags or Sig 320 mags, you might want a conversion kit that fits in the mag well.
If you are made of money and simply want one lower for a dedicated caliber, there are plenty of options just for you. Some of these run on common mags, others have had special mags created for them.
This one is still elusive. There are AR-like shotguns. There are shotguns that have similar controls. Some of those will even allow for the end-user to swap out grips and stocks with other AR stocks. Still, when you pop the hood, it is easy to see where the modularity ends.
The most economical of these can be found in the Rock Island Armory VR series. https://armscor.com/firearms/ria-imports/shotgun-series/shotguns-mag-fed-series/
AR-15 Lower Receivers: Materials
99% of the AR-15 lower receivers in the wild, if not more, are aluminum. Aluminum is exceptionally durable, reasonably light, and inexpensive. It is more readily available and easier to work than steel and is also reasonably resistant to corrosion.
The cheapest lowers are cast. A mold, slightly over-sized, is filled with aluminum which will then be cut down to final dimensions by a CNC machine. These are the bottom of the price-point and—in the minds of many—100% sufficient.
The argument against a cast aluminum lower is based on the strength of the alternative.
All metals are, at some point in their life-span, cast. Most metals can be work-hardened, though, to increase strength. A cast or an AR-15 lower can be hammered and compressed to increase its strength. And that’s what happens with forged lowers.
A forged lower looks somewhat like a casting. The crystalline structure derived from the hammering, though, makes the material stronger. These forgings are not yet firearms, in the traditional sense, as they have to be milled out, so they can be produced and shipped to gun manufacturers without any hassle.
Billet Aluminum for AR-15 Lower Receivers
A lower cut from a billet looks exactly like a lower cut from a casting until you put it under the microscope. The crystalline structure of most castings is not as compact and strong as a piece of aluminum that has been cut from raw stock that is designed for strength.
The process, though, creates a tremendous amount of waste. While cast lowers are trimmed to size, billet lowers are literally carved from a block. That creates tool-wear, and waste (which is no longer as precious as it was in its initial form). While that is recycled, the return is lower than the initial investment.
Expect a billet AR-15 lower to cost more than a forged lower.
Is there a difference?
For most of us, no. That doesn’t mean we don’t want the best we can get. If you want the best, you’ll pay for it. If you want an economical option that will work, that’s available, too. The choice, as in all DIY projects, is yours.
Maybe a better question would be “who do you believe?” Some companies that cut their own billet lowers claim that a forging is risky. If there is a flaw, or an occlusion present before the forging, it can weaken the end product.
I think it only fair to note that the same holds true for lowers cut from a billet. A flaw is a flaw is a flaw.
Those who specialize in billet lowers will try to sell you on their process. The same holds true for forged lower makers. While the decision may be made based on how an individual receiver looks, I’ve not personally noted a single difference in how one or the other performed.
Types of aluminum for AR-15 Lower Receivers
While you may not care about the first steps used to shape your AR-15 lower receivers, I would suggest investing in decent materials. When it comes to aluminum, all metals, in fact, there are numerous industry standards that grade expected performance outcomes.
Almost all of the lowers being made now are made from 7075 T-6 aluminum. Some folks still try to get by with 6061 T-6. It isn’t as hard and is more prone to elastic deformation.
When I’m looking at metallurgy, I tend to trust reputable makers and question those who are trying to bring a gun to market at the absolute bottom end of the spectrum. Why? Maybe I’m gullible. But there’s another reason….
Who makes the aluminum? Who makes the gun?
There are some gun makers who start with the basics and make their own guns. Few of those are AR makers. Instead, most companies simply buy pre-shaped forgings or castings from companies that have the expertise to deal with molten metals.
While there are more than 200 companies in the US making AR-15s, most of them get their parts from about 10 suppliers. This is even true for barrels. We, the buying public, will often brag about one company and bad-mouth another, without realizing that the aluminum came from the same factory, and was sent to the same middleman who milled it, and was simply stamped with a logo and assembled by the last company inline—the one we swear loyalty to.
This goes back to the modularity we were talking about earlier. These companies don’t make parts, they simply assemble them.
Steel for AR-15 Lower Receivers
Why not cut an AR-15 lower receiver from steel? It is obviously possible. It has been done. The main reason steel isn’t used is related to weight and tool wear.
If you are serious about a steel AR-15, check out Turnbull’s (very) limited edition guns. The company notorious for meticulous restorations of 19th-century firearms applied the same finish details to the AR. Turnbull made an 8620 carbon steel AR that they color case hardened with bone charcoal. With its walnut furniture, it is certainly a unique gun.
Now we’re getting into some seriously contentious issues. The lower of an AR doesn’t really take a beating. While the barrel, of course, is heating up and cooling down, and the upper receiver has some fast-moving parts, the lower is just holding the gun together.
The most intense forces on the AR-15 lower receiver comes from seating the magazine and from recoil. Is plastic strong enough?
Possibly. Some companies are sold on the idea. Tennessee Arms makes a nylon lower reinforced with some metal parts.
Polymer80 makes an 80% lower that you finish yourself. It is a polymer.
American Tactical’s Omni was one of the first to be widely available.
Let’s be clear about this. Plastic and nylon are not as resilient as aluminum. Aluminum isn’t as resistant as steel. Steel isn’t as tough as Titanium, which is also available.
If you could find enough adamantium, someone would make an AR-15 lower.
AR-15 Lower Receivers: Design Options
Almost every AR parts manufacturer has a unique twist on the basic design. In some cases, this is just cheeky commentary on the safety selector. Others might round over some of the sharp edges.
I personally prefer to match my AR’s lower to its upper. If you get parts made by the same company, odds are that the aesthetics will be similar.
Integral Trigger Guard
Another bragging point some companies claim is the integral trigger guard. This is the thinnest portion of the lower, which means it is difficult to cut. It also restricts an element that can be customized by the end-user.
If you don’t have a trigger guard built-in, you can opt for one that’s flat, or slightly bent, or way oversized (for use when wearing winter gloves).
Skeletonized AR-15 Lower Receivers
If you’re into weight-saving extremes, you might like the skeletonized lowers. These are just like normal AR-15 lowers, only they’ve had key elements removed to form something that looks like a ribcage of sorts.
This Ghost Rifles’ lower skeletonizes the mag well, but not the area that protects the trigger group.
How much weight will you save? It depends on who does the cutting. You still have to have structural strength, of course. The downside is that these holes allow grit and grime into the inside of the lower. While this happens to all ARs, skeletonized ARs are wide-open. They remain popular for run-and-gun competitions but haven’t really gained any serious ground for tactical use.
Most aluminum lowers—most aluminum parts, for that matter—are anodized. This is an electrolytic process that provides a protective oxide layer resistant to abrasion, dirt, grime, oils, etc.
Type II anodizing and type III are both common. Type III is harder and more expensive, but few can tell the difference between the two with the naked eye.
If you’d rather go for something more colorful, there are Duracoat options. This is a strong, spray-on paint that can be easily applied at home. For those who want something harder still, Cerakote is a ceramic based finish that has very high heat and wear resistance.
Really Out There?
When I build ARs, I typically shop around. I prefer forged lowers in a simple black anodized finish. That’s me. I’m not flashy.
There are some flashy lowers. Some, like the Sharps Brothers’ The Jack, is really unique. The mag well on this one is shaped like a skull. The teeth of the upper jaw appear to eat the magazine when inserted.
There are other options. If you don’t want a skull, you can get a mag well shaped like a Spartan’s helmet.
AR-15 Lower Receivers: Parting Shots
There are two other ideas here that are important to any discussion of AR-15 lower receivers. As I’ve run through this long introduction, I’ve made one basic assumption. If you end up buying a stripped lower, you’ll need to add the parts yourself. That’s not too complicated.
If you don’t want to bother, I’d recommend something like this complete lower from PSA. If money is no object (or if you don’t trust your skills), a complete lower saves time and energy.
There’s another assumption that I made, one that is much more advanced. If you are really into the DIY culture, you may be ready to cut your own lower. Lowers that are designated as “80%” are, by law, not yet firearms. You have to finish the other 20%, though, for it to work as a gun. We’ll take a deep dive into this topic, too.
For now, you have some decisions to make. What’s it going to be?