AR-15 Receiver Specifications
Mil-spec dimensional callouts, aluminum alloy properties, forging process comparison, and MIL-A-8625 Type III hard anodize requirements for AR-15 upper and lower receivers.
Mil-Spec Dimensions
Core dimensions from the M16A2 and M4 Technical Data Packages.
| Dimension | Value | Source |
|---|---|---|
| Takedown / pivot pin diameter | 0.250 in (small-pin mil-spec) | M16A2 TDP |
| Large pin diameter (Colt SP1) | 0.315 in | Colt legacy drawings |
| Trigger / hammer pin diameter | 0.154 in | M16A2 TDP |
| Large trigger pin (Colt) | 0.170 in | Colt commercial |
| Buffer tube thread | 1.185-16 UN-2A | M4 Carbine TDP |
| Pivot to takedown pin center distance | 6.375 in | M16A2 TDP |
| Magazine well width | 0.905 in nominal | M16A2 TDP |
7075-T6 vs 6061-T6 Aluminum
7075-T6 is the mil-spec aluminum for forged M16 and M4 receivers. 6061-T6 appears on some commercial billet and cast receivers and offers better corrosion resistance but significantly lower strength.
| Property | 7075-T6 | 6061-T6 | Note |
|---|---|---|---|
| Ultimate tensile strength | 83,000 psi (572 MPa) | 45,000 psi (310 MPa) | 7075-T6 is roughly 84% stronger in tension. |
| Yield strength (0.2%) | 73,000 psi (503 MPa) | 40,000 psi (276 MPa) | Resistance to permanent deformation under load. |
| Brinell hardness (500 kg) | 150 HB | 95 HB | Harder surface resists dings and scuffs. |
| Density | 0.101 lb/in³ (2.81 g/cm³) | 0.098 lb/in³ (2.70 g/cm³) | Essentially identical weight for identical geometry. |
| Elongation at break | 11% | 12% | 6061 is slightly more ductile before fracture. |
| Primary alloying element | Zinc (5.6%) | Magnesium and silicon | Zinc drives the strength increase in 7075. |
Source: Aluminum Association Standards and Data, ASM Handbook Volume 2 (Properties and Selection: Nonferrous Alloys), and matweb.com published material data sheets for 7075-T6 and 6061-T6.
Forged vs Billet vs Cast
Forged
Hot aluminum billet pressed between dies under several thousand tons of force, refining grain structure along load paths. Mil-spec lowers and uppers are forged 7075-T6.
Pros
- Best fatigue life of the three
- Grain structure aligned with stress paths
- Lowest per-unit cost at scale
- Meets military TDP requirements
Cons
- Limited to standard M16 profile
- Surface finish requires additional machining
Billet
Machined directly from a solid block of 7075-T6 or 6061-T6 on a CNC mill. Offers design freedom beyond the forged envelope.
Pros
- Custom profiles and integrated trigger guards
- Tighter dimensional tolerances possible
- Cleaner cosmetic finish before anodize
Cons
- Grain structure not aligned with load paths
- Higher cost per unit
- Strength depends entirely on the source alloy
Cast
Molten aluminum poured into a mold. Usually 6061 or proprietary casting alloys. Rare in modern quality receivers.
Pros
- Lowest manufacturing cost
- Can produce complex shapes in one step
Cons
- Porosity and inclusion defects
- Lower strength than forged or machined billet
- Does not meet mil-spec
- Avoided by reputable builders
Type III Hard Anodize (MIL-A-8625)
Mil-spec receivers use MIL-A-8625 Type III Class 2 anodize. Type III produces a thick, dense oxide layer integral to the aluminum surface, much harder than the decorative Type II sulfuric acid anodize found on consumer goods.
| Property | Value | Source |
|---|---|---|
| Standard | MIL-A-8625 Type III Class 2 | MIL-A-8625F |
| Typical coating thickness | 0.002 in (50.8 microns) | MIL-A-8625F Table I |
| Minimum thickness | 0.0005 in (12.7 microns) | MIL-A-8625F |
| Surface hardness | 60 to 70 HRC equivalent | Aluminum Anodizers Council data |
| Salt spray resistance (Class 2 dyed black) | 336 hours per ASTM B117 | MIL-A-8625F Section 3.6.3 |
| Abrasion resistance (Taber) | Max 3.5 mg/1000 cycles weight loss | MIL-A-8625F Section 4.5.7 |
| Class 1 | Undyed (natural) | MIL-A-8625F |
| Class 2 | Dyed (typically black on receivers) | MIL-A-8625F |
Tolerance Classes
Mil-spec TDP drawings call out dimensions with explicit plus / minus tolerances. Critical-to-fit features (pin holes, buffer tube threads, magazine well width) are typically held to plus-or-minus 0.002 to 0.005 in. Non-critical cosmetic features may be held to plus-or-minus 0.010 in or looser.
"In spec" means every dimensional callout on the drawing, every material callout, every surface finish callout, and every process callout meets its stated tolerance. Partial conformance does not qualify. A receiver that meets dimensional tolerances but uses unspecified 6061 aluminum is not mil-spec.
Disclaimer: Specifications are sourced from published military technical data packages, SAAMI standards, and manufacturer data sheets. Always verify critical dimensions with calibrated measuring tools.