AR-15 Bolt Carrier Group Specifications
Mil-spec dimensional callouts, steel grades, testing procedures, coating comparison, and gas key staking requirements for AR-15 bolt carrier groups.
Mil-Spec BCG Dimensions
| Callout | Value | Source |
|---|---|---|
| Bolt body diameter | 0.7495 in (+0.0000 / -0.0005) | M16 TDP |
| Carrier key screw (Grade 8) | Torque 35 to 40 in-lb | TM 9-1005-319-23&P |
| Gas key staking | Two staked impressions per screw, staked into screw head | TDP gas key assembly callout |
| Firing pin protrusion | 0.028 in to 0.036 in | M16 armorer gauge |
| Bolt lug count | 7 radial lugs plus extractor recess | M16 TDP |
Bolt Steel: Carpenter 158 vs 9310
| Steel | Composition | Use | Properties |
|---|---|---|---|
| Carpenter 158 | 0.13% C, 3.25% Ni, 1.0% Cr, 0.12% Mo, 0.55% Mn | Mil-spec bolts (M16, M4 TDP) | Excellent fatigue life, case-hardenable, toughness specified for cyclic bolt stress. |
| 9310 | 0.10% C, 3.25% Ni, 1.2% Cr, 0.12% Mo | Aerospace, some commercial bolts | Similar toughness to C158. Case hardens deeper. Not the mil-spec steel but physically adequate. |
| 8620 | 0.20% C, 0.55% Ni, 0.5% Cr, 0.2% Mo | Some low-cost commercial bolts | Less expensive than C158 or 9310. Lower fatigue life. Avoid for duty builds. |
| Tool steel (S7, various) | Varies | Boutique bolts | Premium marketing claim. Real world performance depends on heat treat, not grade. |
Source: Carpenter Technology published 158 alloy data sheet, AISI 9310 spec, and Colt M16A2 TDP bolt material callout.
Why Carpenter 158 is mil-spec
Carpenter 158 was selected for the original M16 bolt in the 1960s because of its combination of case-hardenability, core toughness, and consistent heat-treat response. The TDP locks it in as the required steel. 9310 is arguably as good or better in most metallurgical measures, but "mil-spec" means conformance to the TDP callout, and the TDP says 158.
Testing: MPI and HPT
HPT (High Pressure Testing)
What it tests: Bolt and barrel are fired with a proof cartridge loaded approximately 25 to 30% above normal maximum pressure (roughly 70,000 psi for 5.56 NATO).
Why it matters: Proves the bolt and barrel can contain a severe overpressure event. Required by mil-spec and by most NATO militaries.
MPI (Magnetic Particle Inspection)
What it tests: Bolt is magnetized and coated with iron particles; surface and near-surface cracks concentrate the particles and become visible under UV or white light.
Why it matters: Catches cracks that may have initiated during forging, heat treat, or HPT. Every mil-spec bolt must pass after HPT.
Shot-peened
What it tests: Bolt lugs are bombarded with small steel media to induce beneficial compressive stress in the surface.
Why it matters: Delays fatigue crack initiation on lug roots, the most common failure location.
Source: ASTM E1444 (Standard Practice for Magnetic Particle Testing), SAAMI proof load specifications, and US Army M16 TDP bolt acceptance criteria.
Coating Comparison
Published hardness, friction, and salt spray hours for common BCG coatings. Salt spray per ASTM B117 neutral salt fog.
| Coating | Hardness | Friction (μ) | Salt spray | Cleaning | Cost vs phosphate |
|---|---|---|---|---|---|
| Mil-spec Phosphate | Substrate HRC (not the coating itself) | 0.12 to 0.15 | 96 to 200 hours (oiled) | Hardest to clean. Porous surface holds carbon. | Baseline |
| Nitride / Melonite / QPQ | 65 to 72 HRC surface | 0.10 | 500 to 1,000 hours | Easy. Dense, smooth surface. | +15 to 25% |
| Nickel Boron (NiB) | 65 to 70 HRC | 0.05 to 0.08 | 350 to 500 hours | Very easy. Wipe off. | +40 to 75% |
| DLC (Diamond-Like Carbon) | 70 to 90 HRC (up to 9,000 HV) | 0.05 to 0.10 | 500 to 1,000 hours | Easy. Low-surface-energy carbon. | +75 to 150% |
| TiN (Titanium Nitride) | 80 to 85 HRC (approx 2,400 HV) | 0.4 to 0.5 (dry) | Moderate, depends on substrate | Easy. Smooth ceramic surface. | +50 to 100% |
| Hard Chrome | 66 to 72 HRC | 0.16 | 200 to 500 hours | Easy. | +30 to 50% |
Source: Poeton Industries, Oerlikon Balzers, and UCT Coatings published technical data sheets. ASTM B117 salt spray results from vendor lab reports. Real-world corrosion resistance depends on substrate prep and thickness.
Full-Auto vs Semi-Auto Carrier
| Aspect | Full-auto (M16) | Semi-auto (AR-15) |
|---|---|---|
| Weight | ~11.6 oz | ~11.3 oz |
| Rear shroud | Full-length under firing pin tang | Cut back, exposes hammer-to-carrier contact earlier |
| Legal | Legal to own and install in a semi-auto rifle (the carrier itself is not a machine gun part) | Legal in all configurations |
| Function | Slightly delays unlock on full-auto trip; marginal difference on semi | Designed for semi-auto only |
Legal note: ATF has confirmed (Technology Branch letters) that an M16-profile carrier in a semi-auto AR-15 is legal. The auto sear contact surface is in the lower receiver, not the carrier.
Gas Key Staking
Mil-spec gas keys are attached to the carrier with two Grade 8 fasteners torqued to 35 to 40 in-lb and then staked. Staking deforms the carrier material into the screw heads so the screws cannot back out from cyclic gas pressure and vibration.
Proper staking
- Two distinct stake marks per screw (four total)
- Displaced material contacts the flat of the screw head
- Torque specified before staking (35 to 40 in-lb)
- Stake marks deep and crisp, not decorative
Improper staking
- Shallow "dots" that do not touch the screw head
- Single mark per screw
- Loctite used as a substitute for staking
- Visible gap between key and carrier flat
Source: M16 TDP gas key assembly drawing and TM 9-1005-319-23&P (Unit and Direct Support Maintenance Manual, M16/M4).
Disclaimer: Specifications are sourced from published military technical data packages, SAAMI standards, and manufacturer data sheets. Always verify critical dimensions with calibrated measuring tools.