If you are planning on upgrading your M4 or you’re building a new M4 from the ground up, the lower receiver is one of the most important components to invest in. Almost all configurations of an M4 lower receiver on its own are already considered a firearm by federal law.
However, don’t let that intimidate you. If you’re a beginner or a casual firearm enthusiast, we’ve got everything you’ll need to know before purchasing your first M4 lower receiver. If you’ve already got an idea of what you’re looking for feel free to skip to our buyer’s guide or head to the Best M4 Lower Receivers.
|PSA AR-15 Complete Lower Receiver
|$120 Shop NowClick to read my review
|BRN-15 M4 Stripped Lower Receiver
|$70 Shop NowClick to read my review
|KE-15 Stripped Lower Receiver
|$126 Shop NowClick to read my review
|AR-15 M4E1 Stripped Lower Receiver
|$100 Shop NowClick to read my review
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Is It an AR-15 or an M4?
You’re always going to hear about the M4 or the AR-15, whether from friends, the community, or the news. Eventually, someone’s going to ask which is which, and at first glance, you might not be able to tell either. I don’t blame you.
The AR-15 and the M4 are pretty much the same guns. It makes sense as both belong to the same family. In fact, the M4 was created as the carbine version of the M16, while the M16 is the military version of the AR-15. However, what does that even mean?
Well for starters, at a barrel length of 14.5″, the M4 is a carbine-length rifle while the AR-15 has a 16″ standard barrel. The stock barrels on an M4 are also adjustable while the AR-15 comes with a fixed stock. But the most significant difference between the AR-15 and the M4 is the select-fire capabilities. The M4 can be fired in either 3-round bursts or fully automatic while the AR-15 is semi-automatic only.
However, these differences are so minute that you can technically add an AR-15 upper to an M4 lower and vice-versa, and shoot without any issues.
Is It Legal to Own an M4?
I’m going to talk about the United States federal law. However, you’re going to have different local state laws since some states, like California, have certain restrictions to owning semi-automatic and carbine rifles.
Currently, there are two schools of thought on owning an M4. If you’re talking about the original Colt M4 Carbine, then the answer is no. The M4 carbine is a military and police-only-issued firearm. However, the term M4 has become generalized over the years to essentially mean an AR-15 built on an M4 style configuration.
This means that under the NFA and United Stated Law, the barrel of an M4 style rifle had to be welded and pinned to a flash hider. It should also meet the 16″ minimum barrel requirement to be legal for civilian use.
What’s a Lower Receiver?
When talking about rifles and firearms, the lower receiver is the frame that has integral internal components such as the trigger mechanism, safety controls, and holding magazine. On an M4 or AR15, the lower receiver is the sole serialized component of the firearms. Hence, by United States Federal law, certain types of lower receivers are considered as true firearms and will fall under certain state and federal jurisdictions.
Parts of a Lower Receiver
There are many components that make up a lower receiver. However, for the purpose of this section, I’m going to talk about all the lower part kits that make up a complete lower receiver.
In order to fire your M4, you must have a trigger assembly in your lower receiver. Without it, you’ll be unable to shoot at all. The trigger assemble contains the trigger, pins and springs, the casing, and everything else.
As the name suggests, the magazine catch does the important job of hooking onto a smell slit to hold the magazine in place. A gun without a magazine catch is essentially worthless since the lower receiver wouldn’t be able to hang on to any magazines. Imagine having a rifle that can’t hold any bullets.
The grip is what allows you to shoot your gun in a correct manner. There are many types of grips to choose from. For instance, standard AR-15’s and M4 grips often come pre-installed onto your rifle’s lower receiver. Just make sure your grips are compatible with your state’s law. For example, California has a strict prohibition against using pistol grips on M4 and AR-15 rifles.
It’s also known as the Buttstock, shoulder stock, or butt. The stock is what you press against your shoulder in order to gain stability when firing your weapon. The stock also houses the spring and the buffer tube, both of which are needed to fire your weapon as well.
Stocks come in many designs and materials to choose from. Depending on your build, you can opt for a fixed stock, or go tactical with a folding or adjustable stock option.
Finally, the lower receiver housing is the main piece of the lower receiver that houses all of the other components. You can purchase housings made from a variety of materials depending on your build priority. A lightweight build will opt for a polymer housing while someone looking to shoot .50 BMG’s might want an aluminum or stainless steel housing.
Complete, Stripped, or 80% Lower receivers
Complete Lower Receivers
The complete lower receivers are the whole package lower receivers. Simply put, they come with everything you need — all the parts, already assembled. All you need is to attach a complete upper and you’re ready to go. It’s for this reason that complete lower receivers, like stripped lower receivers, are considered by US law, a firearm.
This part is important since complete lower receivers are considered by United States Federal law as legal firearms. Therefore, you will need to comply with federal and state sales restrictions. If you’re looking to buy a new complete lower receiver, you’re going to need an FFL and the shop will need a record of your FFL before any transaction can take place, even when you’re ordering online.
If you’re more budget-conscious or a beginner shooter, a complete lower receiver will work fine for you. They’re technically cheaper than purchasing each lower part kit separately and don’t come with anything unnecessary or showy features like custom trigger pulls.
Stripped Lower Receivers
When talking about stripped lowers, we usually talk about the actual lower receiver housing itself without any other internal components installed. Stripped lowers are usually for higher-end guns and experienced shooters who want to customize their M4 to fit their needs to a tee.
If you buy a stripped lower receiver, it’s your responsibility to purchase the other components. So this way, you can add your favorite grip or a personalized trigger, it’s the perfect way to start an M4 build.
People usually approach stipped lower receiver assembly one of two ways. First, if you have the tools and experience, self-assembly at home. The other option and what we suggest is to purchase the components you want and have a professional gunsmith install them for you.
Aside from customization, the main advantage of buying a stripped lower receiver is that it’s ready to go out of the box. It’s almost like purchasing a new barrel where it works once you’ve installed all the other internal parts. And unlike other bare receivers like an 80% lower receiver, it has a CN- machined finish with the proper holes and a precise fit.
Just remember that even without the other lower part kits, a stripped lower receiver is still considered a firearm by United States Federal law and will need an FFL.
80% Lower Receivers
80% lower receivers, also known as unfinished receivers. They are technically incomplete versions of stripped lower receivers. The reason for the 80% is to establish the notion that they’re unfinished lower receivers that still need to be milled out before they can be of any use.
It’s for this matter that 80% lowers are NOT considered legal firearms by the United States Federal. That means there are little to no regulations when it comes to purchasing an 80% lower receiver. No background checks and no need to go through an FFL to purchase one for yourself. Also, depending on your state, they may not be legally required to have a serial number on them, making them practically untraceable.
Of course, the biggest caveat is that you have to finish the receiver yourself before it becomes functional. If you’re building your rifle from scratch, possess the proper tools, willing to put in the labor, and are well-experienced with gun assemblies, this might be the build path for you. However, for beginners, you’re either going to need a really good gunsmith or you should think about sticking to complete receivers instead.
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Other Classifications of the Best M4 Lower Receivers
Forged lower receivers are easy to find and are the most common type of lowers you’ll find in the market. It’s essentially a block of metal, usually aluminum, that’s heated to extreme temperatures. Afterward, it’s hammered into the shape of a lower receiver. This heat hammering process is what binds the aluminum molecules together making them light, strong, and durable.
However, this heat hammering process will often create imperfections on the lower receiver that have to be taken off. As a result, this can cause imperfections in the fit and finish of the lower. To know if you’ve ended up with a faulty receiver, you’ll be able to see some gap issues once you’ve attached your lower to your upper receiver.
Billet Lower Receivers
Billet lower receivers will start out as a solid aluminum billet that is put through a CNC machine to mill out the final design. The milling process is what gives billet lowers their precise dimensions ultimately leading to a tight fit between the upper and lower receivers.
A tight-fitted lower receiver can make your firearm more accurate and will make it harder for debris and outside conditions to enter your rifle. The milling process also makes billet lowers customizable. Hence, allowing you to add a personal touch to your rifle giving and giving it a unique look.
Unfortunately, this milling process also means that billet lowers are going to be less durable than forged lowers. So make sure that the billet lower you end up with is made from the highest quality aluminum. A 6061-T6 aluminum will suffice. But a 7075-T6 aluminum will give you more than the strength of the 6061.
You should also expect to pay more for a billet lower than a forged or polymer lower.
Polymer Lower Receivers
Polymer lower receivers, as the name might suggest, are lower receivers built out of high-quality hardened polymer. High polymer beads are heated, melted, and injected into prefabricated molds to create the lower receiver. This process results in lowers that are extremely light without sacrificing the durability of the lower. All for a fraction of the price of steel and aluminum counterparts.
The number one question I always get about polymer M4 receivers isn’t the cost, or the fit of the lower, but the durability. And the fact is, most companies will always look to switch polymer parts to cut manufacturing costs and increase their profits. However, the idea that polymer firearm parts aren’t durable can be largely appointed to the perceived cheapness and weakness of typical hard plastic we see or use in our everyday lives.
Think about it this way. When the Glock first came out, a handgun with a mostly polymer build, it received a lot of pushback and weariness from the community for being mostly plastic. Fast forward to today, Glocks are now one of, if not the most popular handguns in the world.
Buyer’s Guide to the Best M4 Lower Receivers
With so many aftermarket lower receivers to choose from, you’re probably asking yourself, what makes a great lower receiver?
Read our criteria below to save time and money when purchasing the best M4 lower receivers for your firearms.
Types of Lower Receiver
First, think very carefully about the type of lower receiver that you’ll need for your build. The safest options will always be some variation of a complete lower receiver. Complete lowers are guaranteed to save you time and money and will work well for most scenarios.
Again, keep in mind that your receiver’s weight, durability, and cost will be affected by the type of receiver you use.
Investing in a high-quality lower receiver for your M4 is essential if you want that rifle to endure firing bullets for many years. For a lower receiver, 7075 aluminum combined with a hard coat anodized finish is generally referred to as the gold standard. Aluminum is both sturdy and light, while anodized coatings are very rust and corrosion resistant, guaranteeing long-term endurance.
You should always look for a receiver that has the durability or weight that you need for your build. If you’re looking to use high-powered ammunition or fire a lot of rounds out of your M4, it’s best to choose heavier materials that will make your rifle last longer.
Stripped or Complete
Always keep in mind that you’ll eventually have to choose between a lower receiver that’s been stripped down to its bare components or one that’s pre-assembled and ready to go. Do you want a lower that you can further personalize, or do you want a lower that requires little to no maintenance?
Choose a receiver from a well-known, reliable brand with a loyal following of customers who are confident in the quality of their products. You can find out more about a product by reading reviews. With any firearm, you want to be confident with the parts you’re using. The last thing you want is your M4 falling apart during a self-defense scenario.
Best M4 Lower Receivers for 2022
Check out our list of the best M4 lower receivers below. Even if you don’t end up using one of them, they’ll serve as excellent starting points for your M4 build.
Palmetto State Armory — PSA AR-15 Complete Lower Receiver: Best complete M4 lower receiver.
Looking for the complete package? Then consider the Palmetto State Armory — PSA AR-15 Complete Lower Receiver. Made from forged aluminum, the PSA AR-15 Complete Lower is built slightly differently from other military specs M4/AR-15 lowers. It should fit nicely with other M4/AR-15 upper receivers thanks to its smooth nickel finish. It also functions well with virtually any kind of common AR-15 ammunition.
This PSA AR-15 lower comes with a much smoother trigger than most standard mil-spec triggers. That already on its own makes this PSA lower a worthy upgrade. Additionally, the PSA AR-15 complete lower is a piece that’s built to last and will work with your M4 build.
Although the price tag is a bit of a pill to swallow, this complete lower receiver comes with everything you need already installed. All you need is an upper receiver and you have yourself a complete M4 style rifle.
- Phenomenal trigger
- Durable, high-quality parts
- Beginner friendly
- Complatible with Most types of M4/AR-15 ammo
- Custom Engraving
- Might break your budget
Brownells — BRN-15 M4 Stripped Lower Receiver: Best stripped M4 lower reciever.
If you’re ready to take on a more advanced, then take a close look at the Brownells — BRN-15 M4 Stripped Lower Receiver. This M4 lower by Brownells is incredibly lightweight and easy to customize. Built using a combination of forged and billet receiver methods, this receiver is forged out of high-quality aluminum. It’s also machine-milled to match traditional M4 military specifications.
Although more of an AR-15/HK416 lower receiver hybrid, the stripped lower design is very close to the real Colt M4 carbine receivers. It works as a solid pick-up for M4 owners. This aluminum stripped lower receiver is finished with a matte black anodized coating for better rust and corrosion resistance and is good to use for common 5.56 rounds.
Although a versatile lower receiver overall, remember that it is a stripped lower receiver. So, you’ll need to have the other lower receiver part kits before you can shoot. You’ll also have to make sure all your parts are compatible with each other as well as your upper receiver.
- Easy to customize
- Anodized matte finish for better rust and corrosion resistance
- Lightweight yet durable
- Very versatile lower receiver
- Needs other lower receiver parts
KE Arms — KE-15 Stripped Lower Receiver: Best budget pick M4 lower receiver.
If you find yourself running on a tight budget but still looking for an affordable build, then the KE Arms — KE-15 Stripped Lower Receiver is an option to check out. An easily affordable forged stripped lower receiver, the KE-15 lower is hammered out of high-quality 7075 aluminum. It also has a class 2 hard coat anodized finish for maximum surface wear and weathering protection.
Constructed to proper M4 military specifications, you can expect the KE-15 to be compatible with all other mil-spec AR lower part kits and upper receivers. This durable and lightweight lower will still need a lot more parts before it can be transformed into a complete lower receiver. But for those looking for budget-friendly builds, this is a great place to start.
- Built to mil-spec standards
- 7075 aluminum construction
- Class 2 hardcoat anodized for maximum durability and weathering resistance
- Best for experienced M4 builders
Aero Precision — AR-15 M4E1 Stripped Lower Receiver
If you’re looking for a top-of-the-line lower receiver, then the Aero Precision — AR-15 M4E1 Stripped Lower Receiver is a great place to start. Forged from a custom aluminum base with a mil-spec look, the AR-15 M4E1 lower is lightweight and durable. It features sleeker and smoother than a lot of other military receivers.
Although categorically a stripped receiver, you get a few extra parts that don’t usually come with stripped lowers. The AR-15 M4E1 comes with an easy install threaded bolt catch roll pin, a flared Magwell opening for fast magazine swaps, and an extra-wide trigger guard to accommodate gloved fingers.
Since the AR-15 M4E1 Stripped Lower Receiver is built to military specifications, you can expect a large array of lower and upper part compatibility. Ultimately, it gives you freedom and versatility on your M4 build.
- Top-of-the-line lower receiver for a reasonable price tag
- Anodized black finish for better rust and corrosion resistance
- Comes with a few extra lower part kits to get a head start on your M4 build
- Large A insignia might not be your style
Overall, our pick from the list of the best M4 lower receivers is the PSA AR-15 Complete Lower Receiver from Palmetto State Armory. It is an excellent all-in-one option. It weighs less than a pound, is very sturdy, and has a better trigger than most mil-spec AR triggers.
However, if you’ve got the experience and are looking for a top-of-the-line receiver then consider the AR-15 M4E1 Stripped Lower Receiver from Aeroprecision.
Any one of the lower receivers that we have covered here today will be a solid choice for your M4 build. If you do decide to go a different route, you can follow the buyer’s guide section that we went over earlier to help you find the best M4 lower receivers for your guns as well.
A few reminders before you go. When you buy a stripped lower receiver to double-check compatibility within your M4 lower part kits, especially with your M4 upper. Also, preplan your build. Finding out ahead of time the type and classification of the lower receiver you go with will influence the outcome, the durability, and the cost of your M4 build. A complete lower receiver will almost always be cheaper than putting together a stripper lower with M4 lower part kits.