How to Build an AR-15 Lower Receiver
Woody's answers are based on published federal and state law. Not legal advice. Consult a licensed attorney for your specific situation.
Building an AR-15 lower receiver requires milling or forging an 80% lower, installing the fire control group, buffer tube assembly, and magazine well components. You need proper tooling and must follow all federal and local laws.
How This Affects Your Build
The lower receiver is the serialized firearm component that houses the trigger mechanism and accepts the upper receiver. Building your own allows customization of trigger components, stock interface, and grip angle. This process requires mechanical aptitude and specialized tools for proper fitment and function.
The Detail
Starting with an 80% lower receiver blank, you'll need to mill out the fire control pocket and trigger guard using a jig system or CNC machine. The critical dimensions include the trigger pin holes at 0.154 inches diameter and the hammer pin hole at the same diameter. These must be precisely located and perpendicular to the receiver's centerline for proper function.
The fire control group installation begins with the trigger assembly, followed by the hammer and disconnector. The trigger spring provides the return force while the disconnector spring ensures proper reset. The safety selector detent and spring must be compressed during installation, and the grip secures the safety detent spring. Each pin should fit snugly without binding the moving components.
The buffer tube threads into the rear of the receiver using a castle nut and end plate assembly. The buffer tube requires proper torque specification to prevent loosening under recoil forces. The buffer spring and buffer weight slide into the tube, with the bolt carrier group compressing this system during firing. The stock or pistol brace attaches to the buffer tube according to your intended configuration.
Magazine well finishing involves ensuring the magazine catch spring and button function smoothly with your chosen magazine types. The magazine release button should have positive engagement without excessive force. Test fitting with empty magazines confirms proper dimensions and feed angle geometry.
Build Impact
Your tooling investment will determine build quality and ease of completion, with proper jigs being essential for consistent results. Material choice between aluminum alloy types affects weight and durability, while finish options impact corrosion resistance and aesthetics.