FAXON DUTY SERIES GUNNER PROFILE 13.9" MID-LENGTH 5.56 NATO AR15 BARREL

Rifle Barrels › Duty Series › 5.56 NATO / .223 › 13.9″ Gunner, Mid-Length  |  Works with: 5.56 BCG • .625″ gas block • mid-length gas tube • 1/2x28 muzzle devices • 5.56 suppressors

Why 13.9″ With Mid-Length Gas

Thirteen point nine inches is the short 5.56 that gives up the least. It sends 55 gr out at roughly 2,900 fps, within 100 fps of a 16″ and close enough that 55 and 62 gr loads still fragment past 150 yards and 77 gr open-tip loads work well past 300. It puts a mid-length gas port 9.66″ from the breech with 4.2″ of dwell past it, which is why this barrel runs on a .086″ port and a standard buffer while an 11.5″ mid-length needs a .110″ port and an adjustable block, and why it stays soft with a suppressor instead of overgassed. And with a 5″ to 6″ can on the muzzle, the upper is about the length of a bare 16″ rifle with a flash hider, which is the balance most suppressed 5.56 builds are chasing. Owners who want a rifle instead of a pistol or SBR pin and weld a full-length muzzle device to reach 16″; the FAQ below covers the math.

The Gunner is the profile that keeps it balanced. A .980″ chamber journal and a Government taper from .675″ to .610″ behind the gas block keep the barrel stiff and give it the heat capacity to shoot strings without walking; a .625″ journal and a .500″ section ahead of it strip the weight that a heavier profile carries at the muzzle for no return. At 1.26 lb the gun balances at the magazine well, which is why more than one owner built this barrel for a spouse or a smaller shooter and reported that it “feels lighter” even when it is not. Owners of this exact barrel report just over 1 MOA with mid-grade ammunition, a 1.2″ group from a lightweight build, and a half-inch group at 50 yards suppressed. If you want the shortest suppressed SBR that still carries mid-length gas, the 12.5″ Gunner is an inch and a half shorter with a slightly bigger port; the 14.5″ Gunner is the M4 length; and the 16″ Gunner is the rifle with no paperwork and no permanent muzzle device.

What “Duty Series” Means

Duty Series is Faxon’s 4150 CMV line: the M4/M16 barrel alloy, salt-bath QPQ nitrided inside and out. 4150 carries about 0.50% carbon plus vanadium, which gives it more strength and wear resistance at temperature than 4140, and nitride adds a hard, corrosion-resistant surface to the bore and exterior without plating anything, and it adds virtually nothing to the surface: a few ten-thousandths of an inch, roughly a tenth the width of a human hair. It is the right combination for a barrel that will be shot hard, carried in the weather, and cleaned when there is time. Faxon’s stainless Match Series is the option when you want the precision-barrel formula; the Duty Series is the one to reach for when the rifle has a job.

What’s in the Box

How It’s Made

Every Duty Series barrel is made start to finish in Faxon’s Greater Cincinnati plant, on Faxon’s machines, to Faxon’s prints. This is the routing for this barrel.

1. Drill and pull-ream 4150 CMV bar is gun-drilled, then pull-reamed from the chamber end to a .2195″ bore held to ±.001″, with a 32 Ra maximum finish checked on a profilometer and a borescope pass for reamer marks.
2. Button rifle A carbide button is pulled through from the chamber end to form six grooves, right-hand, one turn in 8 inches, to a .2250″ groove diameter. Bore and groove are air-gauged.
3. Stress relieve and qualify The rifled blank is stress relieved before it becomes a finished barrel, then the OD is qualified true to the bore so every later cut references the same axis.
4. Turn the Gunner profile Muzzle end first: the .625″ gas journal is turned to .6240–.6247″ and checked with a micrometer on every barrel, held within .005″ of the bore, the .500″ forward section is turned true to the same axis, and the 1/2x28 muzzle is threaded and crowned. Then the breech end: the Government taper, the .980″ chamber journal, and the extension thread.
5. Chamber The 5.56 NATO chamber is cut and checked with a headspace gauge; chamber surfaces are held to a 20 Ra finish.
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. Gas port, extension, pin After nitride, the .086″ gas port is drilled and reamed through the .625″ journal, the M4 extension is torqued to 150 ft-lb and pinned, and the barrel is headspaced with a 5.56 go/no-go gauge.
8. Mark The Faxon shield and order number are laser-engraved on the chamber journal and “5.56 NATO 1-8T” ahead of the gas block.

What Reviewers and Owners Say

Voted Favorite Aftermarket Barrel by Shooting Sports Retailer every year since 2022 — the Gold Award, the top honor in the category. Faxon also took the Tactical Retailer Gold Award in the same category in 2025. Here is what independent testers and buyers have said about Faxon barrels and this one.

5.0 stars across 5 reviews on FaxonFirearms.com. Elsewhere: the 5.56 Gunner family averages 4.6 stars across 68 reviews at OpticsPlanet, the Duty Series 5.56 line holds 4.7 across 25 at MidwayUSA, and the Gunner line, 13.9″ included, has fifteen owner reviews at Brownells.

Build It Out: Faxon Parts That Fit This Barrel

Everything below is made or specified to the same prints this barrel is, so the gas system, extension, and muzzle line up the first time. Pick one from each row and the upper is done.

Duty Series 13.9″ Gunner 5.56 FAQs

Is this a 5.56 NATO or .223 Remington chamber?

5.56 NATO, headspaced with a 5.56 go/no-go gauge after the extension is installed. A 5.56 chamber has the longer throat that lets it fire both 5.56 NATO and .223 Remington safely; the reverse is not true, so a .223-chambered barrel should not be fed 5.56.

Why mid-length gas on a 13.9″ barrel?

Because it puts the port about two inches farther from the chamber than carbine-length gas, so the system taps the bore at lower pressure. The port sits 9.66″ from the breech with about 4.2″ of barrel past it, which is more dwell than any carbine-gas 5.56 has and more than the 11.5″ or 12.5″ mid-lengths, so Faxon can ream the port to .086″ and the barrel runs 5.56 NATO on a standard carbine buffer with no tuning. The result is a softer, more linear recoil impulse, less wear on the bolt and buffer, and far less back pressure and gas at your face with a suppressor. This barrel needs a mid-length gas tube.

What is the Gunner profile?

Faxon’s hybrid contour: a full-diameter chamber journal and a Government taper behind the gas block where the steel does the most work, then a .750″ step, a .625″ gas journal, and a .500″ section toward the muzzle where mass adds weight without adding stiffness. At 13.9″ it weighs 1.26 lb, close to a Pencil in weight and closer to a Government profile in rigidity and heat capacity, and it takes the same .625″ low-profile block as every 5.56 Gunner. It is the profile Faxon is best known for.

What bullet weights does the 1:8 twist stabilize?

Everything from 55 gr FMJ through 69 and 77 gr match bullets. 1:8 is the most versatile twist for a general-purpose 5.56; 1:7 is only needed for the longest tracer and 80-plus grain projectiles, and 1:9 starts to run out of spin above 69 gr.

What gas block and gas tube does it take?

A .625″ gas block on the 1.9″ journal and a mid-length gas tube. Faxon makes a .625″ adjustable and a .625″ fixed low-profile block, and a 304 stainless mid-length gas tube. The gas port is reamed to .086″.

What does 4150 CMV mean?

Chrome-moly-vanadium steel with about 0.50% carbon, the alloy the M4 and M16 barrel specification calls for. The higher carbon and the vanadium give it more strength and wear resistance at temperature than 4140, which matters most on a barrel that will see fast strings and sustained fire.

Is the nitride inside the bore?

Yes. Salt-bath QPQ nitride is a diffusion treatment, not a plating, so it hardens the surface of the bore and the exterior while adding virtually nothing to it: the surface grows by a few ten-thousandths of an inch, .0002–.0004″, roughly a tenth the width of a human hair or about the diameter of a single red blood cell. It also gives the barrel its corrosion resistance and matte black finish.

Does it come with a gas block, gas tube, or muzzle device?

No. It ships with the black-oxide M4 extension installed, torqued, pinned, and headspaced. Gas block, gas tube, and muzzle device are sold separately; the muzzle is threaded 1/2x28.

Will it work with a suppressor?

Yes. The 1/2x28 muzzle takes any 5.56 direct-thread can or mount, and mid-length gas is the friendlier system for suppressed use. For a gun that runs suppressed most of the time, pair it with the Faxon .625″ adjustable gas block so you can bring the gas down.

Do I need an adjustable gas block?

Not to run it unsuppressed: with 4.2″ of dwell the .086″ port is sized so the barrel cycles 5.56 NATO and full-power .223 with a standard carbine buffer. You want one if the gun will wear a suppressor, because a can adds back pressure and mid-length gas is the system that benefits most from turning it down, and you want one if you switch between light and heavy loads. Faxon’s .625″ adjustable gas block fits the 1.9″ journal with room to spare.

How fast does 5.56 leave a 13.9″ barrel?

Roughly 2,850 to 2,950 fps with 55 gr ammunition and 2,750 to 2,850 with 62 gr, depending on the load, or about 100 fps less than a 16″ and 100 more than a 12.5″. That keeps 55 and 62 gr loads above fragmentation velocity past 150 yards and 77 gr open-tip loads working well past 300, and owners of this barrel report just over 1 MOA with mid-grade ammunition.

Why 13.9″ instead of 12.5″ or 14.5″?

Dwell and balance. Over the 12.5″, the extra 1.4″ buys about 100 fps and half again as much dwell past the port, so the 13.9″ runs a smaller port and is the more forgiving of the two with a can. Against the 14.5″, it is the same gas system with the port in the same place and six-tenths of an inch less barrel, so a suppressed 13.9″ is that much shorter and lighter at the muzzle for about 25 fps. Pick the 12.5″ if overall length with a can is the priority, the 14.5″ if you want a short device pinned for a rifle, and the 13.9″ if you want the most dwell and the best balance in a sub-16″ 5.56.

Can I put a 13.9″ barrel on my AR?

A barrel under 16″ makes the firearm either a pistol or a short-barreled rifle under federal law. Build it on a lower that has only ever been a pistol, or on a lower registered as an SBR with an approved Form 1 or Form 4, and check your state law before you buy. Faxon cannot advise on your specific configuration.

Can I pin and weld a muzzle device to make it a 16″ rifle?

Yes, and some owners do. A rifle needs a barrel of at least 16″ measured from the closed bolt face to the end of a permanently attached muzzle device, so a 13.9″ barrel needs a device that adds at least 2.1″ beyond the muzzle, pinned and welded or high-temp silver soldered in place; a standard A2 adds about 1.75″ and will not get there, so it takes a full-length, fixed-body flash hider or suppressor mount. Measure with the device seated before it is welded and give yourself margin over 16″. Once it is done the device is part of the barrel and cannot be swapped, and a direct-thread suppressor is off the table unless the pinned device is a mount. Faxon’s MuzzLok devices use a locking extension nut and are not designed to be pinned and welded.

Is the gas journal dimpled for set screws?

No. The journal ships round so any .625″ low-profile, clamp-on, or pinned gas block fits the 1.9″ journal. A set-screw block holds on the nitrided journal when its screws are torqued with thread locker; if you want dimples, mark them through the block’s screw holes and drill.

Where is it made, and what is the warranty?

Made in Greater Cincinnati, Ohio, from bar stock to finished barrel, and covered by the Faxon Lifetime Guarantee.

Dimensions are from the Faxon engineering print and inspection sheets for this SKU; weight is nominal. Gas block, gas tube, and muzzle device sold separately. Owner reviews are quoted as written from FaxonFirearms.com product reviews; press quotes link to their sources. Award: Shooting Sports Retailer Choice Awards, Favorite Aftermarket Barrel, Gold. Covered by the Faxon Lifetime Guarantee.