Complete AR-15 platform breakdown for professional gunsmiths. Upper and lower assemblies, gas system configurations, specifications, and common service procedures.
The AR-15 direct impingement platform is the most modular and widely serviced rifle in America. Its split upper/lower architecture, standardized mil-spec dimensions, and extensive aftermarket ecosystem make thorough platform knowledge essential for any gunsmith shop. This breakdown covers the complete system from bolt to buffer tube.
Upper Receiver Assembly Components
The upper receiver houses the barrel, bolt carrier group, charging handle, and forward assist. Mil-spec upper receivers are machined from 7075-T6 aluminum with an anodized finish and precise internal dimensions that ensure interchangeability. The barrel extension threads into the upper at 1.185"–18 TPI. The star chamber is the critical fit surface that aligns the barrel extension with the bolt's locking lugs.
The bolt carrier group is the heart of the cycling system. The bolt itself contains seven locking lugs that engage the barrel extension's corresponding recesses. Inspect lugs for cracks or peening at every 5,000-round service interval. The gas key must be properly staked to the carrier—a loose gas key causes cycling failures. The firing pin must move freely in the carrier's firing pin channel and the firing pin retainer pin must be present and correctly seated.
Lower Receiver Assembly and Fire Control
The lower receiver is the serialized firearm component and houses the trigger group, pistol grip, buffer tube, and stock. Mil-spec lower receivers are machined from 7075-T6 aluminum with a Type III hard-coat anodize. Critical dimensions include the trigger and hammer pin holes (0.154" mil-spec) and the trigger guard roll pin hole. Enlarged pin holes indicate high round count and may require bushing or receiver replacement.
The mil-spec trigger group consists of five moving parts: trigger, hammer, disconnector, trigger spring, and hammer spring. Factory mil-spec triggers typically pull 5.5–8.5 lbs. The disconnector catches the hammer during automatic cycling to prevent unintended automatic fire. Function check every trigger installation: safety blocks hammer, hammer falls when safety is disengaged, disconnector catches hammer when trigger is held and hammer is manually recocked, reset produces an audible/tactile click.
Gas System Configuration and Timing
Gas system length determines the pressure curve that drives the bolt carrier rearward. Pistol-length (4" gas tube) generates maximum bolt velocity and requires heavier buffers. Carbine-length (7") is the most common configuration on 16" barrels. Mid-length (9") provides a smoother pressure curve on 16"–18" barrels. Rifle-length (12") is used on 20" barrels and produces the lowest bolt carrier velocity for the smoothest cycling.
The gas port in the barrel must be sized appropriately for the gas system length and barrel profile. An oversized gas port causes overgassing—brass ejected at 1–2 o'clock, bolt bounce, and excessive wear. An undersized port causes undergassing—failure to lock open, weak ejection at 5–6 o'clock, short stroking. Gas port size ranges from 0.062" (pistol-length, suppressed) to 0.093" (rifle-length, unsuppressed).
Technical Specifications
| Gas System | Tube Length | Typical Barrel Length | Gas Port Size Range | Buffer Weight |
|---|---|---|---|---|
| Pistol | 4" | 7"–10.5" | 0.062"–0.075" | H2 / H3 |
| Carbine | 7" | 14.5"–16" | 0.075"–0.087" | H1 / H2 |
| Mid-Length | 9" | 16"–18" | 0.079"–0.090" | Standard / H1 |
| Rifle | 12" | 20"–24" | 0.087"–0.093" | Rifle standard |
AR-15 vs AR-10 / Large Frame Distinction
The AR-15 (small frame) and AR-10/SR-25 (large frame) share operating principles but are dimensionally incompatible. Large frame receivers are wider and taller to accommodate .308 Winchester and similar cartridges. Barrel extensions, bolt carriers, and magazines are completely non-interchangeable. The receiver extension (buffer tube) threads differ between platforms. Always verify which platform is on the bench before ordering parts—mixing small and large frame components produces failures that can be dangerous.