Professional gunsmith guide to AR-15 lower receiver dimensions, FCG assembly sequence, timing verification, and troubleshooting.
Lower Receiver Specifications and Dimensional Requirements
Mil-spec AR-15 lower receivers are machined from 7075-T6 aluminum. The fire control pocket — the cavity housing the trigger, hammer, and disconnector — measures 1.250" in length, 0.500" in width, and 0.685" deep from the receiver's rear face. Trigger and hammer pin holes are the critical dimensional features: trigger pin holes require 0.154" diameter; hammer pin holes require 0.154" diameter on standard mil-spec lowers. Some commercial lowers use 0.170" large-pin holes — verify before ordering FCG components.
The receiver extension (buffer tube) threads into the lower at 1.146"–16 TPI. Minimum thread engagement is 0.750". Buffer tube installation requires the correct torque on the castle nut (35–39 ft-lbs) and proper staking of the castle nut to the endplate to prevent it backing out under recoil. A castle nut that backs off changes buffer tube tension, affects stock wobble, and can eventually cause the tube to crack.
Fire Control Group Components and Assembly
The mil-spec AR-15 FCG comprises five moving parts: trigger, hammer, disconnector, trigger spring, and hammer spring — plus two pins. The trigger spring is a double-torsion unit that wraps around the trigger pin with legs bearing against the lower receiver walls, returning the trigger forward after each shot. The hammer spring wraps around the hammer pin with its legs bearing downward against the trigger pin — this is the most commonly mis-installed component in the AR FCG.
Assembly sequence: Install the trigger first, with the trigger spring's front legs seated on each side of the fire control pocket floor. Insert the trigger pin from the right side until it just contacts the disconnector hole. Place the disconnector on top of the trigger, compressing the disconnector spring between them. Push the trigger pin through to capture the disconnector. Now install the hammer with the hammer spring legs straddling the trigger pin — the legs must bear on the trigger pin, not on the receiver walls. Drive the hammer pin from the right side to complete the assembly.
Common installation errors: hammer spring legs installed forward of the trigger pin (causes light hammer strikes), trigger spring legs not seated on the receiver floor (causes hard reset), disconnector spring omitted or inverted (causes failure to reset or hammer follow).
| Component | Specification | Tolerance | Service Note |
|---|---|---|---|
| Trigger pin hole diameter | 0.154" | +0.002" / -0.000" | Oval holes = frame replacement |
| Hammer pin hole diameter | 0.154" | +0.002" / -0.000" | Same as trigger pin |
| Buffer tube thread | 1.146"–16 TPI | Class 2A fit | Min 0.750" engagement |
| Castle nut torque | 35–39 ft-lbs | ±2 ft-lbs | Stake after torque |
| Trigger pull (mil-spec) | 5.5–8.5 lbs | ±0.5 lbs | Verify with trigger gauge |
Timing Verification and Function Testing
With the FCG installed and the upper and lower mated (no buffer or spring required for this test), perform the four-step function check: (1) Safety ON, pull trigger — hammer must not fall. (2) Safety to FIRE, pull trigger — hammer must fall. (3) Keeping trigger held rearward, manually cock the hammer. Release the trigger slowly — you must hear and feel a distinct reset click before the trigger reaches full forward travel. (4) With the trigger forward of reset, pull again — hammer must fall. Any failure in this sequence requires disassembly and diagnosis before proceeding to live fire.
Verify the disconnector function specifically: with the trigger held rearward and the hammer manually cocked, the hammer should be caught by the disconnector, not the sear. If the hammer is caught by the sear (you can confirm this by feeling resistance to further rearward trigger travel), the disconnector is not functioning, and the rifle will not achieve proper semi-automatic cycling.
Common Issues and Professional Troubleshooting
Trigger reset failure on AR-15s most commonly results from one of three causes: trigger spring legs not properly seated against receiver walls, disconnector spring too weak or installed inverted, or a drop-in trigger cassette that is not fully seated in the fire control pocket. Verify spring seating visually before replacing parts. A properly seated trigger spring produces a smooth, clean reset with a tactile click.
Hammer follow — the hammer following the BCG forward without the trigger being released — indicates worn disconnector nose, insufficient disconnector spring tension, or FCG components not at mil-spec dimensions. This is a safety failure and requires immediate correction. Never return a rifle with confirmed hammer follow to a customer.