FAXON DUTY SERIES PENCIL PROFILE 10.5" CARBINE-LENGTH 5.56 NATO AR15 BARREL

Rifle Barrels › Duty Series › 5.56 NATO / .223 › 10.5″ Pencil, Carbine-Length  |  Works with: 5.56 BCG • .625″ gas block • carbine-length gas tube • 1/2x28 muzzle devices • 5.56 suppressors

Why the Pencil Profile on a 10.5″

On a short barrel every ounce is at the front of the gun, and a 10.5″ pistol or SBR gets carried, slung, and held up one-handed more than any rifle does. The Pencil profile is the original lightweight AR contour, and Faxon’s version removes everything the barrel does not need to shoot straight: a .980″ chamber journal for the extension and barrel nut, a body that tapers from .598″ to .500″ behind the gas block, a short .750″ step and a .625″ journal 1.1″ long for a low-profile block, then .500″ to a .615″ muzzle shoulder for the thread. What is left weighs 0.90 lb, about three ounces less than Faxon’s 10.5″ SOCOM, and on a build that is chasing five pounds those three ounces are the difference.

Every barrel opens its group as it heats; a thin one simply gets there sooner, because it has less mass to soak up the heat. The same low thermal mass is why a Pencil cools faster than a SOCOM once you let it sit. What a Faxon Pencil does not do is walk. Point of impact stays put and it does not throw stringers as it warms, because the rifled blank is stress relieved before it is ever turned to this profile, so the thin sections have no locked-in stress to release, and the OD is machined true to the bore so the barrel expands evenly instead of bending. Every barrel is also 100% wet magnetic particle inspected, which finds the surface and near-surface flaws a thin wall cannot afford. It is not a benchrest profile. It is a barrel that lets you build a sub-five-pound 5.56 pistol and still hit what you aim at, and for a truck gun, a bag gun, or a smaller-framed shooter, that is the right trade. If the gun’s job is mag dumps and training days, the 10.5″ SOCOM is three ounces away with far more steel where the heat is.

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 Pencil 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 rear taper from .598″ to .500″, 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 .081″ 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 8 reviews on FaxonFirearms.com. Elsewhere: Faxon’s 5.56 Pencil barrels average 4.9 stars across 47 reviews at OpticsPlanet and 4.9 across 9 at Brownells, this SKU is 5.0 at Primary Arms, and the Duty Series 5.56 line holds 4.7 across 25 at MidwayUSA.

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 10.5″ Pencil 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 carbine-length gas on a 10.5″ barrel?

Because it is the combination that has run on the Mk18 for twenty years. Carbine gas puts the port about 7.7″ from the breech, which leaves roughly three inches of barrel past the port for the bullet to travel while the gas system fills. That dwell is what lets a 10.5″ cycle reliably with 5.56 across ammunition and temperature, suppressed or not. Pistol-length gas on a barrel this long would tap the bore too early and overgas the gun; mid-length would leave almost no dwell. Carbine is the right system for a 10.5″ 5.56, and this barrel needs a carbine-length gas tube.

What is the Pencil profile, and is it rugged enough for a pistol or SBR?

The lightest AR-15 barrel contour: a .625″ gas journal with the body tapered to .500″ on both sides of it. At 0.90 lb it is Faxon’s lightest 10.5″ 5.56 barrel. It is more rugged than its profile suggests because the blank is stress relieved before it is turned thin and every barrel is 100% wet MPI inspected, so there is no locked-in stress to shift under heat and no hidden flaw in the wall. Like any barrel its group opens as it heats, and with less thermal mass it gets warm sooner and cools sooner; what it does not do is shift point of impact or string shots. It is not a benchrest profile, but it shoots MOA with ammunition it likes and is the right barrel for a lightweight pistol or SBR, a truck gun, or a smaller shooter’s build.

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 1.1″ journal and a carbine-length gas tube. Faxon makes a .625″ adjustable and a .625″ fixed low-profile block, and a 304 stainless carbine-length gas tube. The gas port is reamed to .081″.

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, and keep it light. The 1/2x28 muzzle takes any 5.56 can or mount, but a heavy suppressor on a Pencil barrel puts the weight right back where the profile took it off. Faxon recommends the Harmonix Ion 5.56, a titanium can, for Pencil builds. A 10.5″ is the barrel that benefits most from one, because it turns a concussive SBR into something you can shoot indoors with ear protection; for a rifle that runs suppressed most of the time, pair it with the Faxon .625″ adjustable gas block so you can bring the gas down.

Can I put a 10.5″ 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.

How fast does 5.56 leave a 10.5″ barrel?

Roughly 2,600 to 2,750 fps with 55 gr ammunition and 2,500 to 2,600 with 62 gr, depending on the load, or about 400 fps less than a 16″. That is plenty for 5.56 to do its work inside 150 yards; for the best terminal performance from a short barrel, pick loads designed for it, such as 77 gr OTM or bonded 62 gr.

Is the gas journal dimpled for set screws?

No. The journal ships round so any .625″ low-profile, clamp-on, or pinned gas block that is 1″ long or shorter fits the 1.1″ 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, or buy this barrel with the gas block already pinned.

What muzzle device should I run on a 10.5″?

A flash hider or a suppressor. A 10.5″ 5.56 produces a large fireball and a lot of concussion with a bare muzzle or a brake; the MuzzLok flash hider tames the flash and doubles as a suppressor mount, and a suppressor solves both problems. Save the brake for a barrel that is not being fired next to other people.

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.