Purpose of the Lower Receiver
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.
To fully understand this you have to understand all the components of an AR-15 that rely on the LR. (See list below) Each one of these items attaches or interacts with the LR in some form or fashion. If you want to learn about these individual components then please see our other articles, but for the purpose of this article, we will be first talking about the required markings on any legal AR-15 LR.
AR-15 Components Relying on Lower Receiver:
- Buffer Assembly
- Upper Receiver
- Bolt Carrier Group
- Lower Parts Kit
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 the only three markings that must be on a legal LR 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 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.
Semi-Automatic vs. Full-Automatic
In order to create a full-automatic 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.