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The Guide to AR-15 Lower Receivers

Author: Chuck Y.

So, you want to make the big jump and build your own AR-15. There is a daunting array of parts you can use, and you’re going to have to make a choice between all of them. But first, start with the most important part of all, and where any build begins: the lower receiver.

Why is the lower the most important? Simple – it’s the part that holds all the other bits and pieces that make the gun go “BANG”. Your trigger, safety, magazine catch and bolt catch, plus the recoil system. Most of the essential controls are housed in the lower – meaning you can’t do much without it.

The lower receiver is so important that the ATF considers it the “firearm”, the part that must be serialized, and what you’ll receive a background check for. Without that serial number, it’s one of the dreaded “ghost guns” (more on that a little later). All the other pieces are just accessories – necessary maybe, but accessories just the same.

You will usually see AR platform components including receivers referred to as mil-spec. In a nutshell, mil-spec simply means that they conform to military manufacturing specifications. What does that mean? When you get right down to it, it’s been said that an elephant is a mouse built to mil-spec. As an example, the mil-spec for the M16 rifle, which is the closest relative of today’s AR-15, is 68 dense technical pages and refers to 19 other equally complex specifications, standards, and publications. It’s doubtful if anything on a commercial AR-15 is really mil-spec, so when you read it, you should read “dimensions more or less mil-spec”.

The M4E1 and EPC-9 may look similar, but they are purpose-built for different calibers and magazines.

So how do you choose a lower receiver? Should be easy, right?

Maybe not. When you start diving into specifications, you’ll see materials like billet vs. forged aluminum. 6061-T6 or 7075-T6 aluminum. Even injection-molded polymer, which we’ll address last. What about finish? What’s the difference between Raw, Anodized, and Cerakote? After that, there’s the level of completion. Do you know the difference between an 80%, stripped, or complete lower? Caliber is a concern too; how do you know you’ll get the right setup for the cartridge you have in mind?

Is your head spinning yet? Let’s try to break it down so you can make an intelligent decision based on your needs, wants, and how much mama is going to let you spend.

Almost everything here will start an argument over some little detail or value judgment. The only important take-away is that, by the end, you’ll have the knowledge to decide what’s important to you.

Before we get started, answer a short question. Which is better: a Jeep, or a Ferrari? The answer seems simple, if what matters to you is looking good and going fast. What if I want to use it to thrash through a mud bog and push down small trees? That might change your decision.

Material

Let’s start with material. A mil-spec receiver will be made of aluminum. Aluminum for AR’s is either 6061-T6 or 7075-T6. The 7075-T6 is much stronger – coming close to the strength of some steels. Mil-spec requires only 7075-T6. So why is 6061-T6 used? Simple – because it’s cheaper to make. It has close to half the strength of 7075-T6, and it’s much easier to machine. It costs less in time and tooling, so of course manufacturers love it.

But wait – isn’t that dangerous? Who would want a weaker firearm? Remember the Jeep? That mil-spec material will give you a firearm that’s suited for actual combat with its associated abuse and neglect. Unless you really need that, the choice of materials for a safe firearm is open. The ultimate strength of a 6061-T6 receiver is absolutely safe and suitable for hunting, self-defense, and range use. It’s just not mil-spec, but most of whatever gun you build won’t be “mil-spec” anyway.

Manufacturing System

Now let’s complicate the decision-making process. Just a little bit.

Do you want billet or forged construction? Mil-spec calls for forged receivers in which a block of aluminum is heated up enough to slightly soften it, and is then hammered or pressed into a two-part mold that’s roughly the shape of the finished receiver. Then, it’s machined to the final dimensions.

On the other hand, Billet receivers start with a rectangular block of aluminum, and excess material is machined away until all that is left is the finished lower receiver. Many billet receivers are made from 7075-T6, but some lower-cost options are made of 6061-T6. This is mostly a cost-saving measure, since the latter needs less time on the machine to be properly milled.

In the forging process, the metallic grain structure in the original blank is stretched and aligned with the receiver shape, resulting in a much higher tensile strength. In the billet process, the grain remains as it was in the original billet, so grains are interrupted in the process of removing material. Tensile strength of billet construction is typically lower than forged.

So forging is better – right? If you need that Jeep, absolutely. That’s why mil-spec receivers are forged. But in most billet designs, safety is taken into account, typically making it a non-issue. That additional tensile strength is only essential in the most extreme environments and use cases.

In addition to strength, a major argument for forging is cost. Because of the typically reduced time spent machining, good “mil-spec” lowers are less expensive than billet lowers.

On the other hand, billet receivers are much more versatile. The manufacturer can incorporate additional material in stress points, while removing an enormous amount of material in unstressed areas to reduce weight without losing strength. Plus, many billet lowers add unique features like flared magazine wells, or ambidextrous controls.

The KE Arms KE-15 lower is machined with an extra-wide magwell and unique profiles, while the Aero Precision X15 has a traditional forged shape.

 

Another option for billet receivers is significantly increasing receiver wall thickness to improve rigidity, and thus accuracy. In this case, the upper receiver must be purchased with the lower to assure a match between upper and lower.

What about cost? Generally, even with the 6061-T6 material, prices tend to be higher due to the significant amount of machining and associated scrap material. The addition of extra machining for lightness and strength can bring the costs higher than most folks are willing to bear.

Level of Completion

The next decision you must make before buying your first component is just how much work do you want to do yourself? 

80% lower

If you want your build to be really yours from the ground up, you may even want to start with an 80% lower. While an 80% lower is legal to have and use (with the exception of some laws particular to state regulations), we typically don’t recommend them for beginners in the hobby. 80% lowers start with what looks like any other lower receiver, except that it does not have the trigger pocket machined out, nor finished holes for the the safety selector and trigger, among other things. With an appropriate jig (currently around $200 – $300) and the fairly inexpensive end mill bit and drills, you can spend 2 or 3 hours and turn that 80% blank into a fully functional lower receiver. Depending on whose system you use, you can either use an appropriate drill press, or even a common router.

Earlier we mentioned the occasional situation in which the receiver does not have a serial number. When you personally convert an 80% receiver into a complete receiver, you are not required to put a serial number on it – at least as of this writing. This 80% configuration has earned the name “ghost gun” from the anti-gun crowd. The ATF is currently considering a rule which would require serialization of such personally produced firearms. Be sure to check on the latest ATF rules and your local laws before starting an 80% project.

Stripped Lower

If the thought of spending your time on machining and legal research isn’t your thing, you can opt for what most builders do: a stripped lower. This is a completely machined lower, but without any of the pieces that make it work (trigger, safety, pins, springs, etc.). The stripped lower is considered a firearm, and has a manufacturer’s serial number, and can only be purchased through a dealer licensed as an FFL (Federal Firearms Licensee). Purchase of a stripped lower requires a background check just as if you were purchasing a complete firearm.

It is interesting to note that often a stripped lower – even a good quality stripped lower can actually cost less than an 80% lower. This is simple supply and demand. The 80% concept has gotten so popular, the suppliers have a hard time keeping 80% lowers on the shelf, so they command a higher price. Plus, factoring in the cost of jigs, tools you might not have, it’s often far easier and economical to go with a stripped lower. And you have a ton of choices in terms of brands, styles, and features.

Complete Lower

If you’re really in a hurry to build your AR-15, or if you really hate handling numerous small parts (about 37 of them), a complete lower may be for you. Assembling a stripped lower can still be tricky, with tiny springs and pieces trying to fly away and vanish somewhere in your work area, and you’ll likely need tools like punches, an armorers’ wrench, and specialized vise blocks. Complete lowers take all the fussing complete out of the equation.

A complete lower can be ready to go to the range within minutes! All you have to do is open the takedown and pivot pins, position your upper assembly and capture it with the pivot(front) pin. Next you rotate the upper into position and secure it with the takedown pin. You’re ready to add a magazine and shoot!

Finish

Now that you’ve settled on what kind of lower you want, you have to consider finish. Most lower receivers ship with a black hard anodized finish. If you want to finish it yourself in some color that means something to you, you can often order your lower unfinished of “raw”. This is usually only available in 80% lowers. The final finish possibility is “Cerakote”, which is a proprietary ceramic finish. It can be applied in many different colors, but receivers are commonly colored in “military” colors like Black, FDE (faded dark earth), Coyote Tan, Olive Drab, etc. Typically a Cerakote finish adds between $20 and $40 to the receiver price.

A raw aluminum finish is pretty uncommon for serialized lowers – but if you’re looking to apply a truly custom finish, it may be an attractive option.

Caliber Fired

This one is simple – at least from the point of selecting a Lower Receiver. If you’re going to fire one of the dozens of rifle calibers available to the AR-15 platform (5.56 NATO, 300 Blackout, 6.5 Grendel, etc.) you’ll need a standard mil-spec lower receiver just as we’ve seen so far. If you’re going to fire pistol rounds (9mm, 45 ACP, etc.), you’ll need a special lower receiver designed for that purpose.

Most receivers will be obvious about what caliber they’re built for. This side-folding lower from APF takes .223/5.56 and more, while the KP-9 lower is made for 9mm, using Glock magazines.

Available Selections

When it comes to availability of lower receivers, there is a giant selection to choose from. For the purpose of this discussion, we’ll ignore cheap imported items and companies so small they can’t be relied on for necessary support and warranty coverage.

Generally, we prefer suppliers who do all their work in-house rather than relying on a sub-supplier. There is one major exception to this. Because of the complexity and size of the required facility, virtually all suppliers rely on one of a few suppliers for the forged blanks prior to final machining.

From our experience, there are a few suppliers whose lowers range from basic economical models to ultra-high-end specialty models, yet still meet our high standards for quality and reliability.

 

This complete lower from Anderson has a stock, grip, and all hardware installed – it’s ready to snap onto an upper and hit the range.

As an example, Anderson Manufacturing intentionally serves the high value, reasonable price niche. Anderson manufactures everything in-house and their quality standards are good enough for most folks in the hobby. Their attention to detail belies their price level. Just as an example, their AR-15 Lower Parts Kits (the 37 or so bits and pieces that go into the lower receiver) come with each part neatly nested in a specific section of a vacuum formed plastic tray, while most suppliers just throw them into a plastic bag. These small trays make it much easier to keep track of those tiny parts while you’re installing them. Stripped Anderson Manufacturing lower receivers are typically less than $100, while their very good complete lower receivers start around $250.

 

Aero Precision offers a variety of lowers, stripped and complete. The M4E1 line is their most advanced AR lower, with numerous improvements and a billet look – even though it’s forged.

Next on the price scale are the excellent products by Aero Precision. The fit and finish of Aero Precision products is very good, and as a bonus, they have added specialized features designed to minimize many of the homebuilders’ headaches. Some areas on these lowers have threads to contain springs and pins, preventing parts from flying around during assembly. Plus, upgrades like receiver tensioning screws and beveled magwells make for rifles that fit and feel far better than their competition. Aero Precision lowers start around $100, and their complete lowers start around $225.

 

Angstadt Arms makes Glock-pattern lowers compatible with a variety of pistol calibers. Their lowers contain a Last Round Bolt Hold Open mechanism, too.

At the high end of the scale are manufacturers like Angstadt Arms, who sell precisely machined AR-15 receivers as matched sets. This brand specializes in pistol caliber AR-15s, feeding from Glock magazines in 9mm, .40 S&W, .45 ACP, and even 10mm. Angstadt Arms makes their products in-house using skilled gunsmiths, and their line is truly a cut above the rest. Angstadt lowers start at around $280, and their complete lowers start at $600.

The ADM Universal Improved Carbine (UIC) lower has numerous improvements for dynamic shooters, with all controls available on both sides of the gun.

Another high-end supplier is American Defense Mfg. (ADM) who make a premium line of specially designed fully ambidextrous lower receivers, which come with proprietary ambidextrous parts already installed. The UIC lower receiver has a completely ambidextrous magazine catch, as well as ambidextrous bolt catch. These lowers can perform any administrative operation quickly, from either side of the gun, making them a favorite for high-end and wrong-handed users alike. ADM lowers start around $350 and complete lowers cost $450 and up, depending on features

The KE Arms KP-15 polymer lower features an integrated stock and grip, plus a widely flared magwell. Compare to the KE-15, with the same magwell costing hundreds of dollars more – and it’s stripped!

KE Arms offers very high-quality billet lowers and high-end complete lower assemblies. While their billet lowers are feature-rich, they sell monolithic injection molded polymer lower receivers, with an integral buttstock and pistol grip. The KP-15 and its pistol-caliber sibling, the KP-9, are tough as nails and surprisingly lightweight. KE Arms offers excellent quality billet lowers starting around $300, with top-end complete assemblies running around $700. The KE Arms polymer receivers start at around $100 stripped. The complete lowers, depending on features, run anywhere from $250 to $550.

Decision Time

You now have all the options, and I’d bet you still don’t know exactly what to buy. Just like everything else in this life, first do what you must, and only then do what you want. Evaluate your needs. What does your gun have to do? Your gun MUST be reliable if its purpose is home or personal defense – no question.. If it’s going to be primarily a range gun, accuracy is important. For be a hunting gun, weight can come into consideration if you’re hiking. For heavy-duty, professional use such as security or combat, durability is necessary.

After refining what you need, you can begin to choose what you want and of course what you can afford. The choice is all yours, and I hope we’ve given you enough here to make that choice wisely. Now go have fun building that gun!

 

One Last Tip

If there’s anyone that knows the AR-15 platform, it’s the US military. As a special offer for our readers, you can get the Official US Army Manual for AR-15/M4/M16 right now – for free. Click here to snag a copy.

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2 thoughts on “The Guide to AR-15 Lower Receivers

  1. You omitted a discussion of options for the FCG pocket. While few, if any “high shelf” stripped lowest are made any more, someone interested in expanding their knowledge base could benefit from learning that difference as well as the fact that there are a few manufacturers, like Centurion Arms & Matrix Arms, that still produce lowers with “true” mil-spec FCG pockets (minus the infamous “3rd hole”) for those that after the most authentic reproduction (minus the prison term).

  2. Thank you, for the in depth information.

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