FAXON DUTY SERIES GUNNER PROFILE 20" CARBINE-LENGTH 350 LEGEND AR15 BARREL

Rifle Barrels › Duty Series › 350 Legend › 20″ Gunner, Carbine-Length  |  Works with: 350 Legend BCG • .750″ gas block • carbine-length gas tube • 1/2x28 muzzle devices • .36 cal suppressors

Why a 20″ 350 Legend, and Why the Gunner Profile

350 Legend exists because Ohio, Michigan, Iowa, Indiana, and a growing list of states let hunters use a straight-wall rifle cartridge where a bottleneck round like 5.56 or .308 is not allowed, and because a straight-wall cartridge built on the AR-15 gives those hunters a light, familiar, low-recoil rifle instead of a shotgun. The cartridge’s published ballistics come from a 20″ barrel, and this is the barrel that produces them: a 150 gr load at about 2,400 fps and a 180 gr at about 2,200, roughly 100 fps and a couple hundred foot-pounds more than a 16″. That margin does not change what the cartridge is, a 200-yard deer round, but it is the margin that keeps a hunting bullet above its expansion velocity at the far end of that 200 yards, and it is why owners who hunt with this barrel say they bought the 20″ to match the box. Faxon’s 350 Legend Q&A answers the build questions.

A .35-caliber bore takes more steel out of a barrel than a .22 does, so this Gunner is built like Faxon’s 300 Blackout Gunners rather than the 5.56 version: a Government taper behind the gas block with a .845″ step at the block, a .750″ gas journal, and a taper to .625″ ahead of it. The chamber journal and body are where a barrel gets its stiffness; the taper is where a barrel gets heavy without getting stiffer, and on a 20″ that is a long taper, which is why the barrel weighs 1.61 lb, four ounces more than the 16″ and lighter than most heavy 16″ 5.56 barrels. A carbine-length gas system with a .098″ port and twelve inches of dwell runs the low-pressure cartridge with a standard buffer and a standard 5.56 bolt, and the 1/2x28 muzzle takes Faxon’s 9mm devices and, through a HUB adapter, its .36-cal Harmonix suppressors. The 16″ 350 Legend is the choice for a rifle that carries through thick timber and gives up only 100 fps; Faxon builds that one into a suppressor-ready 16″ 350 Legend upper if you would rather have it done.

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 .348″ 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 16 inches, to a .355″ 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 .750″ gas journal is turned to .7490–.7497″ and checked with a micrometer on every barrel, held within .005″ of the bore, the Gunner taper ahead of it is turned true to the same axis, and the 1/2x28 muzzle is threaded and crowned. Then the breech end: the Government taper, the .845″ step, the .980″ chamber journal, and the extension thread.
5. Chamber The 350 Legend 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 .098″ gas port is drilled and reamed through the .750″ journal, the Big Bore M4 extension is torqued to 150 ft-lb and pinned, and the barrel is headspaced with a 350 Legend go/no-go gauge.
8. Mark The Faxon shield and order number are laser-engraved on the chamber journal and “350 LEGEND 1-16T” 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 4 reviews on FaxonFirearms.com. Elsewhere: Faxon’s 350 Legend Gunner barrels have twenty owner reviews at OpticsPlanet, and the 16″ holds 5.0 stars at Primary Arms.

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 350 Legend FAQs

Is this a SAAMI 350 Legend chamber, and what bolt and magazines does it need?

Yes, a SAAMI 350 Legend chamber, headspaced with a 350 Legend go/no-go gauge after the extension is installed. The cartridge was designed around the AR-15: it uses the same .378″ bolt face as 5.56, so a standard 5.56 bolt carrier group runs it, and it fits a standard lower. The two things it needs that a 5.56 build does not: 350 Legend magazines, because the straight-wall case will not feed reliably from 5.56 magazines, and this barrel’s Big Bore extension, which has its feed ramps opened up for the .390″ case. Faxon’s 350 Legend Q&A covers the rest of the build.

Why carbine-length gas on a 20″ 350 Legend, and what does it need to run?

Because 350 Legend is a low-pressure, fast-burning straight-wall cartridge that needs its port early to catch enough gas, whatever the barrel length: carbine-length puts it about 7.7″ from the breech, and on a 20″ that leaves more than twelve inches of barrel past the port, the longest dwell of any Faxon 350 Legend barrel. That long dwell is why the port on this barrel is reamed to .098″ where the 16″ needs .109″: the bullet is in the bore longer, so the same pressure has more time to push the carrier. With factory 145 to 180 gr ammunition it cycles with a standard carbine buffer and spring and a standard 5.56 bolt carrier group; an owner runs it with a Faxon 5.56 BCG and reports it feeds perfectly. If a suppressor is in the plan, an adjustable block lets you turn the gas back down. This barrel needs a carbine-length gas tube.

What is the Gunner profile, and why is the 350 Legend version built like the 300 BLK?

Faxon’s hybrid contour: full diameter where the barrel gets its stiffness (the chamber journal and the body behind the gas block) and tapered ahead of the block where mass only adds weight. Because a .35-caliber bore leaves less wall than a .22, the 350 Legend Gunner keeps the same Government taper behind the block but a .845″ step and a .750″ gas journal, where the 5.56 Gunner runs a .750″ step and a .625″ journal. That is why it weighs 1.61 lb at 20″, lighter than most heavy-profile 16″ 5.56 barrels, and why it takes the most common AR gas block.

Why 1:16 twist, and what bullets does it stabilize?

Because 350 Legend shoots short, fat .355″ to .357″ bullets at moderate velocity, and a short bullet needs very little spin to fly straight. 1:16 is the twist the cartridge was specified with and it stabilizes the whole range of factory loads, 145 through 180 gr, without over-spinning a thin-jacketed hunting bullet. From a 20″ barrel the 150 gr loads leave at about 2,400 fps and the 180 gr at about 2,200, which is why the cartridge hits like a .30-30 inside 200 yards.

What gas block and gas tube does it take?

A .750″ gas block on the 1.9″ journal and a carbine-length gas tube. Faxon makes a .750″ adjustable and a .750″ fixed low-profile block, and a 304 stainless carbine-length gas tube. The gas port is reamed to .098″.

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 Big Bore 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 a .36-cal-rated can on the 1/2x28 HUB adapter, never a .30-cal can, and a carbine-gas 16″ straight-wall cartridge over-gasses with a can more than a 5.56 does, so the adjustable block is part of the setup. For a rifle that runs suppressed most of the time, pair it with the Faxon .750″ adjustable gas block so you can bring the gas down.

Is 350 Legend legal for deer in straight-wall states, and how far will it hunt?

It was designed for them. Ohio, Michigan’s southern zone, Iowa, Indiana, and other states with straight-wall rules allow 350 Legend for deer, though every state writes its own case-length and caliber limits, so check the current regulations where you hunt. From a 20″ barrel, 150 to 180 gr hunting bullets leave at 2,200 to 2,400 fps and carry enough velocity to expand and enough energy for deer to about 200 yards, which is where Faxon and the ammunition makers put the practical limit. Inside that range it hits like a .30-30 with less recoil than a .243, from a rifle that weighs what a 5.56 carbine does.

How fast does 350 Legend leave a 20″ barrel, and is it worth it over the 16″?

About 2,400 fps with 145 to 150 gr loads, 2,300 with 165 gr, and 2,200 with 180 gr, depending on the load, or roughly 1,900 ft-lb at the muzzle. Those are the numbers on the ammunition box, because 350 Legend ballistics are published from a 20″ test barrel; a 16″ gives up about 100 fps. Whether the four inches are worth it depends on the hunt: for a stand or blind rifle that shoots to the edge of the cartridge’s range, the 20″ keeps the bullet expanding at 200 yards and steadies the sight picture; for a rifle that gets carried through timber, the 16″ is handier and hits nearly as hard.

Why does my 350 Legend have feeding issues, and how do I fix them?

350 Legend feeds a fat, straight, rimless case through a magazine and receiver designed for a bottleneck 5.56, so feeding is the thing every 350 Legend AR has to be set up for. Three things fix nearly all of it. First, magazines: use magazines made for 350 Legend, whose feed lips and follower are cut for the case; standard 5.56 magazines are the most common cause of nose-dives and double feeds. Second, the extension: this barrel’s Big Bore M4 extension has its feed ramps opened for the .390″ case, and it works with both ramped and non-ramped uppers. Third, the bolt: a standard 5.56 bolt is correct for 350 Legend and a Faxon 5.56 BCG is what owners run; a worn or out-of-spec extractor shows up first on a straight-wall case. Owners who set the rifle up this way report it feeds perfectly.

Is the gas journal dimpled for set screws?

No. The journal ships round so any .750″ 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.