Duty Series Barrels

What “Duty Series” Means

Faxon makes rifle barrels from two steels: 416-R stainless for the Match, MATCH-X, and bolt-action lines, and 4150 CMV for Duty Series. 4150 is the chrome-moly-vanadium alloy the military M4 and M16 barrel specification calls for. It carries about 0.50% carbon, roughly a quarter more than 4140, plus vanadium, and the difference shows up as strength and wear resistance at temperature: a 4150 barrel holds its throat longer through fast strings and hot chambers, which is why the specification requires it. Faxon buys it as certified bar stock and does everything else in-house.

The finish is salt-bath QPQ nitride, applied to the bore and the exterior. Nitride is a diffusion treatment, not a plating: nitrogen is driven into the surface of the steel, so nothing is plated on the bore; the surface grows by virtually nothing, .0002–.0004″, roughly a tenth the width of a human hair or about the diameter of a single red blood cell, and there is no plated layer to wear through or to hide a tool mark under. The result is a surface harder than the steel underneath, black, and corrosion-resistant enough that a wet week in the field is a wipe-down, not a problem. Chrome lining still has its place under the sustained heat of belt-fed and full-auto fire it was developed for; for the vast majority of shooters, nitride does what they need and more, and it needs little to no break-in.

Every Duty Series barrel ships as a finished assembly: the barrel extension is installed, torqued to 150 ft-lb, pinned, and the assembly is headspaced with a go/no-go gauge for its cartridge. AR-15 barrels use an M4 feed-ramp extension in black oxide; .450 Bushmaster, .458 SOCOM, and 400 Legend use Faxon’s Big Bore M4 extension; AR-10 barrels use a DPMS Gen 1 / LR-308-pattern extension; the 9mm and 10mm blowback barrels have no extension at all and seat on a 1.000″ receiver journal with an indexing pin. Bore and groove are air-gauged, the chamber is checked on a profilometer, the muzzle is threaded and gauged, and every barrel is wet magnetic-particle inspected right before it is nitrided. The crown is an 11° target crown.

Choose Your Profile

Profile is the shape of the steel from the chamber to the muzzle, and it is the main thing that separates one Duty Series barrel from the next in the same cartridge and length. Faxon cuts six of them. The right one is a question of how the rifle will be carried and how it will be shot.

The Duty Series Lineup

Forty-one barrels across eleven cartridges. AR-15 barrels use an M4 extension and a standard barrel nut; AR-10 barrels use a DPMS Gen 1 / LR-308-pattern extension; PCC barrels are blowback and use no extension. Gas journals are 1.9″ long on Gunner, SOCOM, Big Bore, and AR-10 barrels and 1.1″ on Pencil and HBAR. All are 4150 CMV, button rifled, QPQ nitrided inside and out, and headspaced.

Weights are nominal. Journal is the gas block diameter; PCC barrels have none. 8.6 Blackout muzzles are M18x1.5 with a tapered shoulder and ship with the taper adapter. Big Bore muzzles are 11/16x24 and take .45-caliber-bore devices only.

Which Duty Series Barrel Is Right for You

A general-purpose 5.56 carbine

The 16″ Gunner is the barrel Faxon is known for: mid-length gas, 1.37 lb, balanced at the magazine well, five-star rated across hundreds of reviews. The 16″ Pencil saves another three ounces; the SOCOM adds heat capacity for a rifle that gets run hard.

A pistol or SBR in 5.56

Faxon builds the short 5.56 five ways. The 10.5″ SOCOM and 10.5″ Pencil on carbine gas are the standard; the 11.5″ carbine-gas Gunner runs any ammunition on a standard buffer; the 11.5″ mid-length and 12.5″ mid-length are the softer-shooting, suppressor-friendly choices with an adjustable block; the 7.5″ SOCOM is the smallest 5.56 Faxon makes.

A 14.5″ or 13.9″ for a 16″-overall rifle

The 14.5″ Gunner, 14.5″ Pencil, and 14.5″ SOCOM put a mid-length gas system on the shortest common carbine barrel; the 13.9″ Gunner is the same idea an inch shorter. All are mid-length gas with a full five inches of dwell.

An 18″ or 20″ rifle

Rifle-length gas is the softest-cycling AR-15 gas system, and the 18″ Gunner and 20″ Gunner give a 5.56 everything it has from a rifle-length tube at 1.58 and 1.67 lb. The 16″ HBAR is the bench and bipod choice.

A suppressed or subsonic build

300 Blackout is the cartridge designed for it. The 7.5″ and 10.5″ Gunners on pistol gas run subsonics and supersonics from a can; the 16″ on carbine gas is the rifle version. All are 1:8, .750″ journal, 5/8x24. The 10.5″ and 16″ 7.62x39 barrels bring the AK cartridge to the AR at .30-caliber cost per round.

A deer rifle for a straight-wall state

The 16″ and 20″ 350 Legend Gunners run on a standard 5.56 bolt with Faxon’s Big Bore upper as the recommended option. The 16″ 400 Legend steps up in bullet weight on a 6.8 SPC / .224 Valkyrie bolt. See 350 Legend vs. .450 Bushmaster for the comparison.

A big bore

The 16″ .450 Bushmaster and the 16″ and 10.5″ .458 SOCOM Gunner Big Bores take Faxon’s .458/.450 bolt carrier and require the Big Bore upper receiver, which is cut for the fat case at the feed ramps and ejection port.

An AR-10

Six .308 Win barrels from the 13.9″ Gunner to the 20″ Gunner, in Gunner and Pencil, 1:10, on a DPMS Gen 1 extension, plus the 8.6 Blackout line at Faxon’s original 1:3 twist in 8″, 12″, and 16″. Read why the Faxon AR-10 barrel and the 8.6 Blackout guide.

A pistol-caliber carbine

Three 9mm and two 10mm Light Tapered blowback barrels, 8.5″ to 16″, with the Glock®-magazine feed ramp machined into the barrel and no gas system to buy or tune. Pair with Faxon’s 9mm or 10mm blowback bolt carrier group.

Unsure? The 5.56 Barrel Guide walks through length, gas, and profile, and the Upper Builder configures a complete AR-15 upper around any 5.56-style Duty Series barrel with the parts that fit it.

Gas Systems and Gas Blocks

Every gas-operated Duty Series barrel is ported for its length and gas system, and the port is reamed, not just drilled, to a size Faxon sets for that barrel: a shorter barrel or a shorter gas system gets a smaller port for its higher pressure, a longer one gets a larger port for its lower pressure, and the goal on every barrel is a rifle that cycles factory ammunition on a standard buffer. The gas block journal sits behind a shoulder so a low-profile block seats in the same place every time; the journal is 1.9″ long on Gunner, SOCOM, Big Bore, and AR-10 barrels and 1.1″ on Pencil and HBAR, so any standard low-profile block fits either.

Gas block diameter follows the profile: .625″ on 5.56 Pencil and Gunner, .750″ on SOCOM, HBAR, 300 Blackout, 7.62x39, 350 Legend, and every .308, .875″ on the big bores and 8.6 Blackout. The gas tube is the length of the gas system: pistol, carbine, mid, or rifle. Faxon’s adjustable gas blocks in all three diameters meter the port with no screw in the gas path and are the right pairing for any barrel that will run suppressed or switch between loads; the fixed low-profile blocks are the right pairing for one that will not.

What Every Duty Series Barrel Goes Through

1. Drill and pull-ream 4150 CMV bar is gun-drilled, then pull-reamed from the chamber end to the bore diameter for its cartridge, held to ±.001″ and checked on a profilometer.
2. Button rifle A carbide button is pulled through from the chamber end to form the grooves (six on most barrels) at the twist for the cartridge: 1:8 on 5.56, 300 Blackout, and 7.62x39; 1:10 on .308 and 9mm; 1:16 on 350 Legend, 400 Legend, .450 Bushmaster, and 10mm; 1:14 on .458 SOCOM; 1:3 on 8.6 Blackout. Bore and groove are air-gauged.
3. Stress relieve and qualify The rifled blank is stress relieved so the profile turning and the heat of use do not move the bore, then the outside is qualified true to the bore so every later cut references the same axis.
4. Turn the profile The chamber journal, the profile, the gas journal and its shoulder, and the muzzle shoulder are turned true to the bore; the muzzle is threaded, gauged, and crowned at 11°.
5. Chamber The chamber is cut to a go gauge and checked on every barrel.
6. MPI and nitride The machined barrel is 100% wet magnetic-particle inspected for surface and near-surface flaws, then salt-bath QPQ nitrided inside the bore and out, hardening the surface while adding virtually nothing to it.
7. Port, extension, headspace After nitride, the gas port is drilled and reamed to the size set for that length and gas system, the extension is torqued to 150 ft-lb and pinned, and the assembly is headspaced with a go/no-go gauge for its cartridge. Blowback barrels get their indexing pin pressed in instead.
8. Mark The Faxon shield, order number, and cartridge and twist are laser-engraved.

What Reviewers and Owners Say

Faxon’s AR-15 and AR-10 barrels have been voted Favorite Aftermarket Barrel by Shooting Sports Retailer every year since 2022 — the Gold Award, the top honor in the category, and it is an award for Faxon barrels as a whole, not one series. Faxon also took the Tactical Retailer Gold Award in the same category in 2025. The 16″ Gunner alone carries hundreds of five-star reviews across FaxonFirearms.com, Primary Arms, OpticsPlanet, Brownells, and MidwayUSA, and Pew Pew Tactical’s reviews of the 16″ Gunner, 16″ Pencil, and 350 Legend barrels are linked from each product page along with that barrel’s own owner reviews.

Duty Series FAQs

What is the difference between Duty Series, Combat Series, Match Series, and MATCH-X?

Duty Series is 4150 CMV chrome-moly-vanadium steel, button rifled, and QPQ nitrided inside and out: the hardest-wearing, most corrosion-resistant barrel Faxon makes and the one built for a rifle that is carried, run hot, and cleaned when there is time. Combat Series is the same 4150 CMV built the classic way, with a chrome-lined bore and a manganese phosphate exterior, in M4, medium taper, and Gunner configurations. Match Series is 416-R stainless with 5R rifling and a nitride finish; MATCH-X is 416-R honed to 8 Ra before rifling and lapped to 16 Ra after, left uncoated. Duty and Combat are the working barrels; Match and MATCH-X are the precision barrels.

Is 4150 CMV better than 4140?

For a barrel, yes. 4150 carries about 0.50% carbon against 4140’s 0.40%, plus vanadium, which gives it higher strength and better wear resistance at the temperatures a barrel throat sees during fast strings. That is why the military M4 and M16 barrel specification calls for 4150 and why Faxon uses it for every Duty Series barrel. 4140 is fine steel for a barrel that is never shot hot; 4150 is the steel for one that is.

Is the nitride finish inside the bore?

Yes. Salt-bath QPQ nitride is a diffusion treatment that hardens the surface of the bore and the exterior while adding virtually nothing to it, a few ten-thousandths of an inch, roughly a tenth the width of a human hair or about the diameter of a single red blood cell, and there is no plated layer to wear through. It gives the barrel its corrosion resistance and matte black finish, and it needs little to no break-in.

Why nitride instead of chrome lining?

Chrome lining still has its place: it was developed for machine guns, holds up best under the extreme, sustained heat of belt-fed and full-auto fire, and some military and agency contracts still specify it. For the vast majority of shooters, including semi-automatic rifles run hard in classes and matches, nitride does what they need and more: it hardens the bore while adding virtually nothing to it, a few ten-thousandths of an inch, it treats the exterior as well, it resists corrosion, and it needs little to no break-in.

Which profile should I choose?

Pencil is the lightest (1.20 lb at 16″ in 5.56) for a carbine that is carried more than it is fired in long strings. Gunner is the balanced middle (1.37 lb) and Faxon’s most popular barrel. SOCOM is the heavy-duty M4 profile (1.73 lb) with the most heat capacity for a rifle that runs suppressed and hot. HBAR is the stiff bench and bipod profile (2.06 lb). Big Bore and Light Tapered are the profiles for their cartridges. Every profile in the same cartridge and length shoots the same ammunition to the same velocity; the difference is weight, balance, and how many rounds it takes to heat up.

What size gas block and which gas tube do I need?

The gas block matches the journal diameter in the product’s specifications: .625″ on 5.56 Pencil and Gunner barrels, .750″ on SOCOM, HBAR, 300 Blackout, 7.62x39, 350 Legend, and .308, .875″ on .450 Bushmaster, .458 SOCOM, 400 Legend, and 8.6 Blackout. The journal is 1.9″ long on Gunner, SOCOM, Big Bore, and AR-10 barrels and 1.1″ on Pencil and HBAR; any standard low-profile block fits either. The gas tube matches the gas system length in the product name: pistol, carbine, mid, or rifle. Blowback 9mm and 10mm barrels use neither.

Do I need an adjustable gas block?

Not for an unsuppressed rifle on factory ammunition: every Duty Series barrel is ported to cycle a standard buffer as-is. An adjustable block earns its place when the barrel will run suppressed, when it will switch between very different loads, or on the short mid-length barrels (11.5″ and 12.5″ mid-length), which carry a large port for their short dwell and shoot noticeably softer with the gas metered. Faxon’s adjustable blocks meter the port with no screw in the gas path.

Is the barrel extension installed and headspaced?

Yes. Every Duty Series barrel ships with its extension torqued to 150 ft-lb, pinned, and headspaced with a go/no-go gauge for its cartridge, so it drops into a mil-spec upper with a standard barrel nut and any in-spec bolt of the right pattern. The 9mm and 10mm blowback barrels have no extension; they seat in the upper on a 1.000″ receiver journal with an indexing pin, headspaced to a go gauge.

Which barrels need a special bolt, upper, or magazine?

5.56, 300 Blackout, and 350 Legend run a standard 5.56 bolt carrier group and standard AR-15 magazines (350 Legend uses its own magazines). 7.62x39 needs a 7.62x39 bolt and magazines. 400 Legend uses a 6.8 SPC / .224 Valkyrie bolt. .450 Bushmaster and .458 SOCOM use Faxon’s .458/.450 bolt carrier and require the Big Bore upper receiver; the Big Bore upper is a recommended option for 350 Legend. .308 and 8.6 Blackout are AR-10 barrels for a DPMS Gen 1 / LR-308 upper and bolt. 9mm and 10mm blowback barrels need a Glock®-pattern PCC lower and Faxon’s 9mm or 10mm blowback bolt carrier.

Can I pin and weld a 14.5″ or 13.9″ barrel to 16″?

The 14.5″ and 13.9″ barrels are made with that in mind: the 1/2x28 muzzle takes any pin-and-weld-friendly device, and a device of the right length brings the permanently attached overall length past 16″. Faxon does not pin and weld barrels or advise on a specific configuration; the work should be done by a gunsmith, and the measurement is yours to confirm. MuzzLok® is a quick-detach suppressor mount and is not intended for pin and weld.

What twist rates do Duty Series barrels use?

1:8 on 5.56 (stabilizes 55 through 77 gr), 300 Blackout (110 gr supersonics through 220 gr subsonics), and 7.62x39; 1:10 on .308 Win and 9mm; 1:16 on 350 Legend, 400 Legend, .450 Bushmaster, and 10mm; 1:14 on .458 SOCOM; and Faxon’s original 1:3 on 8.6 Blackout, which spins the heavy subsonics fast enough to expand on game.

What are Integral Series and Pinned Gas Block barrels?

Both are Duty Series barrels with one more operation. Integral Series barrels have a permanently attached, integral muzzle device; Pinned Gas Block barrels ship with a low-profile gas block pinned to the journal at the factory. They are cross-listed here because they are Duty Series steel and process, and each has its own category with its own details.

Where are Duty Series barrels made, and what is the warranty?

Every Duty Series barrel is made start to finish in Faxon’s plant in Greater Cincinnati, Ohio, from certified 4150 CMV bar stock, on Faxon’s machines, to Faxon’s prints, and is covered by the Faxon Lifetime Guarantee.

Dimensions and process steps are from Faxon engineering prints and inspection routers; weights are nominal. Prices are MSRP and subject to change. Gas blocks, gas tubes, bolt carrier groups, and muzzle devices sold separately. Award: Shooting Sports Retailer Choice Awards, Favorite Aftermarket Barrel, Gold, 2022–2025; Tactical Retailer Choice Awards, Gold, 2025.