Purpose of the Lower Receiver
The lower receiver or LR as I will refer to it throughout the rest of this article is the piece of equipment that holds all of the required components in place for an AR15/M16 to operate properly. To understand the different components, I have broken them out below.
Fire Control Group- This is most commonly known as the trigger, disconnector, pin, and hammer. It is the component that the user pulls to release the hammer and fire the gun. To see the in-depth details please see our other article.
Selector Switch- This component is used to switch the weapon between Safe, Fire or (in the case of a Title 3 weapon) Automatic mode. The selector switch comes in standard right-handed and ambidextrous models.
Bolt Catch- The bolt catch is a component of the lower parts kit. Its function is to stop the bolt’s travel when the magazine is empty and it is pushed upwards.
Magazine Release– The magazine release is another component of the lower parts kit. Its function is exactly what it states, to release and lock the magazine in place.
Magazine Well- The magazine well is a physical part of the LR. It is the place where you insert a magazine. The one thing to note is that many manufacturers have started to create flared magazine wells to allow for easily inserting a magazine. The flared magazine well is not a standard military specification.
Takedown Pins- Takedown pins are the reason the AR-15 platform is so versatile. They allow a user to remove the upper receiver to perform maintenance or replace it with another upper receiver. A user can also replace it with a receiver that supports different calibers as well.
Handgrips- Just as it sounds it is where you hold the rifle to fire. There are numerous options for hand grips on the market so feel free to look at some of our reviews on them.
Lower Receiver Extension – This part is also called the buffer assembly and is comprised of the lower receiver extension tube, recoil spring and buffer. It is also where you attach a stock. To learn more about this please see our detailed article on it.
Upper Receiver- This is too large of a piece to discuss here, but you can see our other article. To understand at a high level, the upper receiver contains the other components needed to successfully fire a round.
If you have not learned already, everything in the AR-15 is centered around the lower receiver (LR.) That is why it is a must that you purchase a quality LR that meets military specifications. There is a reason the military has created formal specifications for the LR. They want the ability to switch between vendors and still use their equipment.
Types of Lower Receivers
When you start researching or shopping around for an LR you will notice that they are referred to as 80%, stripped and complete. At first, this can be confusing, so I have outlined the differences below.
80% LR- When you see an 80% LR for sale it is an LR that is only 80% machined/complete. You are probably asking why you would want to buy one of these? There are a few reasons, first it allows you to practice machining and create your own LR. Second, it is cheaper and provides you with an unregistered LR.
One of the most common questions we get asked is whether it is legal to own an LR that is not registered and does not have a serial number. Technically, it is completely legal, but remember that you are using a receiver that you have machined and if it fails or causes injury than you are at fault. Also, you can not sell an LR that you have made without being a registered FFL dealer and have engraved the proper markings on the LR.
Stripped LR- These are finished lower receivers that have the proper markings and will be transferred via FFL. They do not include any other components other than the LR itself. This is the most common way that distributors sell LRs.
Complete LR- A complete LR will include a lower parts kit, fire control group, hand grip, buffer assembly and a stock. Many people purchase these if they do not want to take the time and build their own LR.
Buying/Selling a Lower Receiver
If you read the post above called “Type of Lower Receivers” then this question has already been answered. In order to purchase an LR, you will need to find a gun dealer and have it transferred to a local FFL dealer. You can find FFL dealers via the BATFE website along with many other gun sites on the web. Remember that if someone tries to sell you LR that they have machined and it does not include the proper markings and they are not a registered FFL dealer than it is illegal. Please avoid this at all costs, as you can be persecuted along with them.
Lower Receiver Manufacturers
To make this short and sweet, please see the forum post below. It gives a broad overview of who the manufacturers are for each gun dealer on the market.
Lower Receiver Markings
Markings on LR’s will vary by manufacturer, many of which will create custom symbols and coloring for their LR to make them stand out. The most important thing to note is that there are only three markings that must be on a legal LR. Those are the serial number, manufacturer name, and city. Once those three markings are applied than an LR can be customized as the owner or manufacturer pleases.
One thing to note is that it is illegal to include copyrighted symbols (i.e. Bushmaster/Colt) without the permission of the company. These markings are protected by copyright and you can be sued for including these on your LR without permission. Although, you will see many LRs where people have included government acronyms or symbols. (i.e. DEA) Technically this is not illegal, but if the BATFE asks and you cannot prove this rifle was not owned by that organization then they can persecute. Does this actually happen? No idea, but better to be safe than sorry.
Lower Receiver Materials
Most LR on the market today are made out of two types of aluminum 6061-T6 and 7075-T6. Although, you will run across polymer LRs on the market. I have listed out the differences below.
6061-T6 vs. 7075-T6: 7075-T6 has twice the tensile strength of 6061-T6. Most manufacturers will only use 6061 to decrease cost. You can see the difference in the strength in the chart below.
|Alloy and Temper||Form||Tensile Strength|
|Note: Data taken from DOT/FAR/AR-MMPDS-01 Metallic Materials Properties Development & Standardization|
Polymer: Polymer has become more and more popular since the release of Glock pistols in 1986. Over the past couple of years, a few manufacturers (ATI, New Frontier) have developed a polymer LR. Since they are very new, there is little information about their durability. A lot of the information on the market is mainly opinion or speculation. If you want to buy a cheap alternative to aluminum its worth a shot, but otherwise go with the standard aluminum LRs.
Forged vs. Billet: This is an age-old argument that you will find numerous opinions on if you take the time to Google or look at forums. In reality, this is not an argument at all. If you look at the chart above you will see that there is little to no difference in strength. The only discernable difference is that because a billet alloy is completely machined it will have cleaner more concise cuts and lines. A forged alloy can also be machined, but the amount of material you can remove is limited, as it has been cast with a specific shape. This will make the lines and cuts not as clean and concise.
Semi-Automatic vs. Full-Automatic
In order to create a full-automatic or title 3 version of the AR-15 also known as the M16 the LR needs to support a device known as an auto-sear. Auto-sears are installed in the back of the LR above the selector switch. An LR needs to be machined with a hole above the selector switch that allows the auto-sear pin to hold the device in place. The LR also needs to have additional space in the back for the auto-sear to drop-in. This space is created when the LR is machined.
A common question that surfaces on the forums is whether you can machine an AR-15 LR to support an auto-sear. This question varies by the LR manufacturer, but generally, you could machine your LR to support an auto-sear. This does not mean that you should. Remember that owning an LR with an auto-sear is illegal and will lead to jail time. This leads to the question of whether you can own either an auto-sear or an LR with the proper machining. Technically you can own either an LR that supports an auto-sear or you can own an auto-sear, but you can not own both at the same time. The general rule of thumb is to avoid owning either of these pieces of equipment. Also, note, that if you have an automatic rifle there are additional modifications to the bolt carrier group and upper receiver, but these will be covered in those articles individually.
As a final note, if you are interested in seeing how this works please visit our gallery pages.