AR-15 Barrel Specifications
Steel types, chrome lining standards, rifling methods, twist rates, chamber specs, velocity by length, and gas port diameters. Sourced from MIL-B-11595E, SAAMI published pressures, and NATO EPVAT data.
Barrel Steel Comparison
| Steel | Composition | Hardness | Use | Notes |
|---|---|---|---|---|
| 4150 CMV (Mil-B-11595E) | 0.48 to 0.55% C, 0.85 to 1.15% Cr, 0.15 to 0.25% Mo, 0.15 min V | 26 to 32 HRC (post heat treat) | Mil-spec M4, M16 barrels | Vanadium grain refinement improves sustained fire performance. |
| 4140 CMV | 0.38 to 0.43% C, 0.80 to 1.10% Cr, 0.15 to 0.25% Mo | 25 to 32 HRC | Commercial duty barrels | Slightly less carbon than 4150. Adequate for typical civilian use. |
| 416R Stainless | 0.15% C max, 12 to 14% Cr, 0.6% Mo, re-sulfurized | 26 to 32 HRC | Match barrels, precision rifles | Machines to tighter tolerances. Not chrome lined. Shorter service life under sustained fire. |
| CM Stainless (410) | 0.15% C max, 11.5 to 13.5% Cr | 26 to 32 HRC | Some premium match barrels | Similar purpose to 416R with better corrosion resistance. |
Source: MIL-B-11595E (Steel, Chrome-Molybdenum-Vanadium, Gun Barrel), AISI/SAE steel composition standards, and Crucible Industries 416R published data sheet.
Chrome Lining (MIL-B-11595E)
Mil-spec barrels are chrome lined per MIL-B-11595E. Hard chrome is electroplated inside the bore and chamber after rifling, providing hardness, corrosion resistance, and reduced fouling at the expense of the last bit of theoretical accuracy.
- Minimum thickness: 0.002 in (50 microns) in bore and chamber
- Hardness: 66 to 72 HRC (hard chrome per AMS-QQ-C-320)
- Barrel life impact: typical mil-spec chrome lined barrel lasts 15,000 to 20,000 rounds with M855 class ammunition; unlined 416R match barrels typically lose match accuracy around 3,000 to 6,000 rounds
- Accuracy trade-off: chrome lining can mask minor bore imperfections, giving roughly 1 to 1.5 MOA vs 0.5 to 0.75 MOA for unlined match barrels
Source: MIL-B-11595E, AMS-QQ-C-320B (Chromium Plating, Electrodeposited), and published round count data from major US armorer manuals.
Barrel Profiles
Nominal weights for representative barrels in each profile. Actual weight varies by length and manufacturer.
| Profile | Typical weight | Contour | Use case |
|---|---|---|---|
| Government (M16A2) | ~2.0 lb (20 in) | Pencil under handguard, thick under FSB forward | Standard M16A2 rifle configuration |
| M4 / M4A1 | ~1.6 lb (14.5 in) | Government profile with M203 cutout | Mil-spec M4 carbines |
| SOCOM (heavy) | ~2.1 lb (14.5 in) | Uniform medium-heavy profile under handguard | Mk18, Mk12 and sustained-fire builds |
| Lightweight | ~1.2 lb (16 in) | Reduced diameter through middle | Patrol, general purpose, weight-sensitive builds |
| Pencil | ~1.0 lb (16 in) | 0.625 in uniform under handguard | Minimum weight, historical A1 profile |
| Bull / Heavy | ~2.5 to 3.2 lb (16 to 20 in) | Straight 0.92 to 1.0 in cylinder | Varmint, match, bench |
| Gunner | ~1.4 lb (16 in) | SOCOM under FSB, lightweight forward | Balance of stiffness and weight |
Rifling Methods
Button Rifled
A carbide button with reverse rifling geometry is pulled or pushed through a drilled bore, cold-forming the lands and grooves.
Pros
- Fast and inexpensive
- Good accuracy on quality steel
- Dominant commercial method
Cons
- Induces stress that must be relieved after forming
- Inconsistent bore dimensions if stress relief is skipped
Cut Rifled
A single-tooth or multi-tooth cutter removes metal from the bore in successive passes to form the grooves.
Pros
- Minimal induced stress
- Most consistent bore on premium match barrels
- Preferred by benchrest and long range shooters
Cons
- Slowest and most expensive method
- Limited high-volume production
Cold Hammer Forged (CHF)
A tungsten carbide mandrel with reverse rifling and chamber shape is placed inside the blank, then hammer dies compress the steel around it.
Pros
- Extremely long service life
- Uniform bore dimensions
- Can chrome line and chamber in one operation
Cons
- High equipment cost limits who can produce
- Steel must be specified for cold forging
Twist Rate Chart
| Twist | Optimal bullet weight | Notes |
|---|---|---|
| 1:12 | 40 to 55 grain | Original A1 twist. Marginal on 62 grain, fails on heavy bullets. |
| 1:9 | 55 to 69 grain | Common commercial compromise. Stable on 69 SMK, marginal on 77. |
| 1:8 | 55 to 77 grain | Most versatile modern twist. Stabilizes nearly all common loads. |
| 1:7 | 62 to 80 grain | Mil-spec M16A2 and M4 twist. Built around M855 62 gr and M856 tracers. |
The Greenhill Formula
Sir Alfred Greenhill published a simple twist rate formula in 1879 that still predicts stabilization requirements for most rifle bullets:
Twist (inches) = (C × D²) / L
- C = 150 for muzzle velocity up to 2,800 fps; 180 for higher velocity
- D = bullet diameter in inches (0.224 for 5.56 / .223)
- L = bullet length in inches
For a 77 grain .224 Sierra MatchKing at 0.998 in long fired at 2,700 fps: twist = (150 × 0.0502) / 0.998 = 7.54 in, meaning a 1:7 or 1:7.5 twist stabilizes it reliably. Greenhill assumes a lead-core bullet; heavy copper solids need a slightly faster twist at the same length.
Chamber Specifications
| Chamber | Leade | Max pressure | Notes |
|---|---|---|---|
| 5.56x45mm NATO | 0.162 in leade (longer) | 62,366 psi max average (NATO EPVAT) | Safe with both 5.56 NATO and .223 Remington ammunition. |
| .223 Remington (SAAMI) | 0.085 in leade (shorter) | 55,000 psi MAP (SAAMI) | Rated for .223 only. Firing 5.56 NATO can spike pressure beyond SAAMI limits. |
| .223 Wylde | Intermediate (close to 5.56) | Safe with both 5.56 NATO and .223 | Designed by Bill Wylde for match accuracy with either cartridge. |
Source: SAAMI "Voluntary Industry Performance Standards for Pressure and Velocity of Centerfire Rifle Sporting Ammunition" (Z299.4) and NATO STANAG 4172 / EPVAT test protocol.
Barrel Length vs Velocity (M855 / 62 gr 5.56 NATO)
| Barrel length | Average MV |
|---|---|
| 20 in | 3,100 fps |
| 18 in | 3,020 fps |
| 16 in | 2,970 fps |
| 14.5 in | 2,910 fps |
| 11.5 in | 2,720 fps |
| 10.3 in | 2,635 fps |
| 8 in | 2,450 fps |
| 7 in | 2,350 fps |
Source: Ballistics by the Inch (ballisticsbytheinch.com) 5.56 NATO M855 test series, and US Army Marksmanship Unit published data. Real-world velocity varies with chamber, lot, and temperature. Rule of thumb: approximately 25 to 50 fps lost per inch removed from a 20 in reference.
Gas Port Diameter by System Length
| Gas system | Barrel length | Typical port diameter | Dwell time |
|---|---|---|---|
| Pistol | 7 to 10.5 in | 0.0785 to 0.093 in | Short. Highest port pressure. |
| Carbine | 10.5 to 16 in | 0.0625 to 0.075 in | Short. High port pressure, harshest cycle. |
| Mid-length | 14.5 to 18 in | 0.0625 to 0.070 in | Moderate. Softer shooting than carbine. |
| Rifle | 18 to 20 in | 0.089 to 0.0935 in | Longest. Lowest port pressure, smoothest cycle. |
Source: Colt M16 / M4 TDP gas port callouts, published BCM, Daniel Defense, and LMT engineering data. Heavy profiles and suppressed builds often reduce port diameter by 0.005 to 0.015 in to tame overgassing.
Disclaimer: Specifications are sourced from published military technical data packages, SAAMI standards, and manufacturer data sheets. Always verify critical dimensions with calibrated measuring tools.