FAXON DUTY SERIES GUNNER PROFILE 20" RIFLE-LENGTH 5.56 NATO AR15 BARREL

Rifle Barrels › Duty Series › 5.56 NATO / .223 › 20″ Gunner, Rifle-Length  |  Works with: 5.56 BCG • .625″ gas block • rifle-length gas tube • 1/2x28 muzzle devices • 5.56 suppressors

Why a 20″ Rifle-Gas Gunner

Twenty inches is the length 5.56 was designed around, and it still does things no carbine does. A 20″ adds roughly 175 fps over a 16″ and 75 over an 18″, which stretches the range at which a 55 or 77 gr bullet stays supersonic, expands, and bucks wind; it gives irons a full-length sight radius; and it carries the rifle-length gas system in its original geometry, with the port 13″ from the breech and more than seven inches of dwell behind it. That is why a 20″ rifle-gas AR shoots flatter and softer than anything shorter, why it is so easy on brass and bolts, and why it is the friendliest host for a suppressor. The service rifle, the SPR’s ancestors, and every M16 ever issued were 20″ rifle-gas guns for these reasons.

The usual price of a 20″ barrel is weight at the muzzle. The Gunner profile is how Faxon avoids paying it: a .980″ chamber journal and a Government taper from .675″ to .610″ behind the gas block, where diameter stiffens the barrel, then a .625″ journal and a .500″ section ahead of it, where diameter only adds swing weight. At 1.67 lb it is lighter than most heavy 16″ barrels and balances like a carbine with a scope on it, and the slim section ahead of the block gives a retro A1 or A2 build the right look. Owners report the same things: sub-MOA groups with ammunition it likes, a point of impact that stays put as it warms because the blank is stress relieved before it is turned, and none of the whip of a true pencil. If you want the same recipe in a handier package, the 18″ Gunner gives up about 75 fps and two inches; the 16″ Gunner is the mid-length carbine.

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–.6250″ 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 .093″ 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 10 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 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 20″ 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 rifle-length gas on a 20″ barrel?

Because this is the configuration the AR-15 was designed as: a 20″ barrel with the port about 13″ from the breech and seven inches of barrel past it. The port sits where pressure is lowest of any AR system and the bullet stays in the bore longest after it opens, so the carrier gets a long, gentle push instead of a short hard one: the bolt unlocks later, the reciprocating mass moves slower, brass comes out easier, and the rifle stays flatter between shots than any carbine, suppressed or not. Faxon reams the port to .093″ for the 20″, a little smaller than the .098″ on the 18″ because the extra dwell makes up the difference, and it runs with a standard rifle or carbine buffer. This barrel needs a rifle-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 .625″ gas journal and a .500″ section toward the muzzle where mass adds weight without adding stiffness. On a 20″ it weighs 1.67 lb, less than most heavy 16″ barrels and far less than a Government-profile 20″, and it takes the same .625″ low-profile block. Ahead of the block it looks like the original A1 barrel, which is why retro builders use it.

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 rifle-length gas tube. Faxon makes a .625″ adjustable and a .625″ fixed low-profile block, and a 304 stainless rifle-length gas tube. The gas port is reamed to .093″.

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 rifle-length gas on a 20″ is the friendliest system there is for suppressed use, with the least back pressure and the least gas at the charging handle of any AR gas system. 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.

How fast does 5.56 leave a 20″ barrel, and what does it buy?

Roughly 3,200 to 3,250 fps with 55 gr ammunition and 2,800 to 2,900 with 77 gr, depending on the load, or about 175 fps more than a 16″ and 75 more than an 18″. That is the longest supersonic range and the most expansion margin 5.56 offers, plus the full rifle sight radius for irons. It is the length the cartridge’s performance was specified from.

Can I run a front sight base for a retro or service-rifle build?

Yes, and this barrel is a favorite for them because the section ahead of the block looks like an A1 barrel. The .625″ journal is 1.9″ long and ships round, not drilled or slotted, so a clamp-on front sight base goes straight on. A pinned, taper-pin front sight base has to be drilled to the barrel as a matched set by a gunsmith, the same way every factory rifle is built; a used base with its old holes is not a match for a new barrel. Faxon does not offer this barrel slotted or drilled. For a rifle-gas build with a low-profile block, use a rifle-length delta ring and handguard or a free-float rail.

Is the gas journal dimpled for set screws?

No. The journal ships round so any .625″ low-profile, clamp-on, or pinned gas block, or a clamp-on front sight base, 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.