What is an AR-15 Lower Receiver?

All firearms consist of three major components: a barrel, an action, and a receiver. The barrel is self-explanatory; the action is simply the gun's moving or mechanical components. The receiver, however, is a little different. It contains the action.

That said, things get a little more complicated when we talk about a gun’s receiver as it pertains to the ATF’s definition of one. This is because the receiver is the serialized part of a firearm, which is required to be transferred through an FFL.

Additionally, there’s no true standardization of what is and isn’t the serialized receiver when a gun like the AR-15, the SIG 550, or the FN FAL has a two-part receiver. On a SIG 550 or an FAL, the upper receiver is considered the de facto receiver in the eyes of the ATF, while our beloved AR-15’s lower receiver is its serialized part. And this is a huge deal, even if it doesn’t seem like it at first.

Why?

Because it means that American shooters can fully customize their AR-15 with new components (other than the receiver) without having to go through an FFL. But this impact of this goes further than just legal ramifications. The fact that an AR-15 is built around a standard lower receiver means that modern AR-15s are the most modular firearms in history. From 410 bore shotguns and 9mm pistol caliber carbines to rimfire precision rifles, Hollywood-quiet 300blk suppressor hosts, and belt-fed range blasters, the only limiting factors to an AR-15’s design are your imagination and your budget. 

But after that last statement, I’m sure you have questions. So, let’s delve into everything you need to know about AR-15 lower receivers.

All mil-spec AR-15 receivers are made of 7075 aluminum, a variety that strikes the perfect balance of hardness and durability. You might be wondering, “Why aren’t AR-15 lowers made of steel like an AK or an M14?” The simple answer is that they don’t need to be.

The lower receiver on the AR-15 isn’t a major stress-bearing component. No moving parts slam into it, and it doesn’t house the barrel or chamber. When designing the original AR-15, Eugene Stoner realized he could make the gun vastly lighter than the competition by using high-grade aircraft aluminum. This makes sense; Eugene came from the aerospace industry and was very familiar with the extensive use of aluminum on aircraft.

But wait, you might be asking, isn’t the upper receiver also made from aluminum? It is. But Stoner reinforced the areas on that receiver that were stress-bearing with steel. For example, the AR-15’s chamber is made out of steel, but is housed in the upper receiver. It is sufficiently strong to contain the round’s detonation without the support of the upper. 

Mostly. If they are labeled as Mil-Spec, then they are, at the very least, dimensionally identical. This means for shooters that all AR-15 components will fit this lower without modification. But just because two receivers are dimensionally the same doesn’t mean they are identical functionally. For example, some receivers are made from billet aluminum instead of the military standard forged. Additionally, some receivers are actually cast from aluminum, zinc, or polymer. 

Forged and billet receivers are the strongest and can withstand tens of thousands of rounds without damage. Cast aluminum receivers tend to be less durable since the aluminum molecules aren’t as uniform inside. Polymer receivers are the least durable, but they are popular because of their ultra-lightweight and total resistance to corrosion.

That all said, there are some specialty receivers that are partially compatible with standard AR-15 components and accessories but differ in very noteworthy ways. For example, there are some companies out there that make dedicated 9mm lower receivers that have a unique magazine well. 

These receivers are purpose-built to function with a specific magazine. Circle Ten makes one that utilizes MP5 magazines, while PSA makes two different 9mm lowers: one uses Glock-pattern magazines and the other CZ Scorpion EVO mags. Both are built to the same material requirements of a military-spec receiver, but they won’t function properly with a 5.56mm upper receiver since the magazines they’re built around only facilitate the use of 9mm cartridges. But let’s go back to a statement I wrote above at the beginning of this section.

While the answer is technically yes, practically, the answer is no. By this, I mean that while an AR-15 lower receiver will eventually wear out after tens of thousands of rounds, the average shooter will never push their gun to that point. In fact, the average competitive shooter who practices constantly and goes to matches every weekend won’t see any real degradation of their lower for years, even if they’re averaging 1,000 round fired a month.

Of course, this is all dependent on how the gun is treated and the assumption that the lower is properly cleaned every thousand or so rounds. Even the gold standard for reliability, the AK-47, will wear itself out quickly if never cleaned and run hard in full-auto.

If you’ve ever shopped for guns online, you’ve no doubt encountered AR-15 lower receivers for sale with the labels stripped and complete - but what does that mean?

When combined with a complete barreled upper receiver, a complete lower receiver ships with every component necessary to function. A shooter simply puts the two receivers together, pushes in the receiver pins, and the firearm is ready to go. But what about stripped lowers?

Stripped lowers are just that - only the lower receiver itself. This means that it will be missing the following components.

• Stock/Brace, depending on whether the gun is a pistol or a rifle.
• Receiver Extension AKA the buffer tube
• Buffer and Recoil Spring
• Pistol Grip (and mounting bolt)
• Lower Parts Kit This includes the following:
  • Magazine Catch Assembly (spring and button)
  • A Safety Selector Lever
  • Bolt Catch Assembly (plunger, catch, spring, and roll pin)
  • Hammer
  • Sear
  • Trigger Assembly (roll pins and spring)
  • Receiver Takedown Pins (front and rear)
  • Trigger Guard Assembly
• Magazine

If this seems overwhelming, take a breath. It’s not nearly as much to install as it seems. In fact, the only specialty tools you’ll need to assemble a stripped lower are an AR-15/M16 armorer’s wrench, a torque wrench, and a punch set. And if you don’t want to drop $50 or $100 on a torque wrench, many automotive stores will let you rent one for a few bucks. On a side note, buy painter’s tape to aid in the assembly of the magazine catch or you will almost certainly scratch your receiver. I’ve put together a few dozen ARs in my life, and even I will scratch the lower doing this unless I take preventative measures.

Why would someone buy a stripped lower? There are a few reasons. The first is that it used to be cheaper to put it together yourself, but advances in manufacturing and the economy of scale with companies like Palmetto State Armory have all but eliminated any savings. (Keep in mind, a mil-spec barebones AR-15 in 2002, before PSA hit the scene, was around $1,000!) 

A more common reason is for shooters to gain a solid understanding of how the AR-15 works. While the development and manufacturing of the AR are very complicated, the assembly of them is not. By putting one together from parts, a shooter will know his/her rifle better than anyone who simply buys a pre-built gun.

The last reason to buy a stripped lower is that a shooter already has specialty parts they want to install, such as a high-end competition trigger or an oversized trigger guard. 

So, what exactly is an AR-15 lower receiver? In the most basic sense, it is a piece of aluminum milled out to accept the trigger assembly, pistol grip, stock, and upper receiver of an AR-15. From a legal perspective, it is the de-facto gun in the eyes of the ATF, and thus must be transferred through an FFL. To shooters, it is the basis for a seemingly endless array of different firearms they can assemble. It may seem like nothing special to a layman, but the AR-15’s modular design owes a great deal of thanks to the humble lower receiver.