FAXON DUTY SERIES GUNNER PROFILE 16" CARBINE-LENGTH 400 LEGEND AR15 BARREL

Rifle Barrels › Duty Series › 400 Legend › 16″ Gunner, Carbine-Length  |  Works with: 6.8 SPC BCG • .875″ gas block • carbine-length gas tube • 11/16x24 muzzle devices

Why a 16″ 400 Legend, and Why the Gunner Profile

400 Legend exists because the straight-wall states gave hunters two AR cartridges to choose from and a gap between them. 350 Legend is light, flat, and soft, and runs a 5.56 bolt; .450 Bushmaster hits like a .45-70 and kicks like one. Winchester designed 400 Legend in 2023 to sit in the middle on purpose: a .4005″ bullet from a widened 6.8 SPC case, so a 215 gr Power-Point leaves at 2,250 fps with 2,416 ft-lb and a 190 gr Deer Season XP at 2,400 fps with 2,430 ft-lb, which Winchester puts at 25% more energy than 350 Legend, more than a .30-30, and twice a 12-gauge slug at 100 yards, with 20% less recoil than .450 Bushmaster and 55% less than the slug. Winchester rates it for deer, hog, and black bear to 300 yards; those published figures are from a 20″ test barrel, and a 16″ gives up roughly 100 to 125 fps and none of the point. Sixteen inches is the length Faxon recommends for a straight-wall AR: it carries through the woods and is legal everywhere a rifle is. Faxon’s straight-wall cartridge guide covers the rules and the choice.

A .40-caliber bore takes most of the steel out of a 1″ blank, so this Gunner is built on the same big-bore contour as Faxon’s .450 Bushmaster and .458 SOCOM barrels rather than the 350 Legend’s: a .980″ chamber journal, a straight .875″ body behind the gas block instead of a Government taper, an .875″ gas journal, and a taper from .750″ to .725″ ahead of it. The body is where the barrel gets its stiffness and where the wall around the chamber and port has to be; the taper is where a barrel gets heavy without getting stiffer. At 16″ the result is 2.00 lb in a rifle that recoils like a .308. A carbine-length gas system with a .109″ port runs the low-pressure cartridge with a standard buffer, and the 11/16x24 muzzle takes Faxon’s MuzzLok .450/.458 brake or a .40-cal-or-larger can, which matters because Winchester loads a 300 gr subsonic for it. If whitetail inside 200 yards is the whole job, the 16″ 350 Legend Gunner is ten ounces lighter and runs a 5.56 bolt; if you want the hardest-hitting straight-wall AR there is, the 16″ .450 Bushmaster Gunner is the same barrel with a bigger hole.

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 .3905″ 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 .4005″ 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 .875″ gas journal is turned to .8740–.8750″ 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 11/16x24 muzzle is threaded and crowned. Then the breech end: the .875″ body, the .980″ chamber journal, and the extension thread.
5. Chamber The 400 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 .109″ gas port is drilled and reamed through the .875″ journal, the Big Bore M4 extension is torqued to 150 ft-lb and pinned, and the barrel is headspaced with a 400 Legend go/no-go gauge.
8. Mark The Faxon shield and order number are laser-engraved on the chamber journal and “400 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.

The 400 Legend is the newest of Faxon’s straight-wall Gunners and has no reviews on FaxonFirearms.com yet; it is rated 4.5 across 2 reviews at OpticsPlanet and is carried at Palmetto State Armory; the 350 Legend and .450 Bushmaster Gunners it shares its extension and process with are rated 5.0 here.

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 16″ Gunner 400 Legend FAQs

Is this a SAAMI 400 Legend chamber, and what bolt and magazines does it need?

Yes, a SAAMI 400 Legend chamber (the cartridge was standardized in January 2023), headspaced with a 400 Legend go/no-go gauge after the extension is installed. Winchester built the cartridge on a widened 6.8 SPC case with a rebated .422″ rim, so it fits a standard AR-15 upper and lower and uses a 6.8 SPC / .224 Valkyrie bolt: Faxon’s 6.8 SPC / .224 Valkyrie bolt carrier group or bolt is the part, and a 5.56 bolt will not work. It also needs 400 Legend magazines (6.8 SPC pattern), because the .440″ case will not feed reliably from 5.56 magazines. This barrel’s Big Bore extension has its feed ramps opened up for the straight-wall case and ships headspaced; Faxon’s Big Bore upper, with its feed ramps and ejection port opened up for straight-wall cases, is the recommended receiver, and a standard mil-spec upper works.

Why carbine-length gas on a 16″ 400 Legend, and what does it need to run?

Because 400 Legend is a low-pressure, fast-burning straight-wall cartridge (45,000 psi SAAMI maximum, against 55,000 for 5.56) that needs its port early to catch enough gas: carbine-length puts it about 7.7″ from the breech with eight and a half inches of barrel past it, and Faxon reams it to .109″, a third larger than a 5.56 port, to make up for the lower pressure there. With Winchester’s 190 and 215 gr loads it cycles with a standard carbine buffer and spring and a 6.8 SPC bolt carrier group. The 300 gr Super Suppressed subsonic makes far less gas, so a gun that runs it, especially with a can, wants an adjustable block. This barrel needs a carbine-length gas tube.

What is the Gunner profile, and why is the 400 Legend version built with a straight body?

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 .4005″ bore leaves little wall in a 1″ blank, the 400 Legend Gunner runs a straight .875″ body and an .875″ gas journal, the same contour as the .450 Bushmaster and .458 SOCOM, where the 350 Legend Gunner runs a Government taper and a .750″ journal. That is why it weighs 2.00 lb at 16″, and why it takes the .875″ low-profile block instead of the .750″.

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

Because 400 Legend shoots short, heavy .4005″ bullets at moderate velocity, and a short bullet needs little spin to fly straight. 1:16 is the twist Winchester specified the cartridge with and it stabilizes the whole factory line: the 190 gr Deer Season XP, the 215 gr Power-Point, and the 300 gr Super Suppressed subsonic. Winchester rates the 215 gr at 2,250 fps and 2,416 ft-lb from a 20″ test barrel and the 190 gr at 2,400 fps and 2,430 ft-lb; from this 16″ expect roughly 100 to 125 fps less, still more energy at the muzzle than a .30-30 and, by Winchester’s numbers, 25% more than 350 Legend.

What gas block and gas tube does it take?

A .875″ gas block on the 1.9″ journal and a carbine-length gas tube. Faxon makes a .875″ adjustable and a .875″ fixed low-profile block, and a 304 stainless carbine-length gas tube. The gas port is reamed to .109″.

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 11/16x24.

Will it work with a suppressor?

Yes. The 11/16x24 muzzle takes a .40-cal, .45-cal, or .458-rated can on an 11/16x24 mount, never a .30-cal or .36-cal can, and a carbine-gas 16″ straight-wall cartridge over-gasses with a can more than a 5.56 does, and the 300 gr subsonic under-gasses, so the adjustable block is part of a suppressed setup. For a rifle that runs suppressed most of the time, pair it with the Faxon .875″ adjustable gas block so you can bring the gas down.

Is 400 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 400 Legend for deer, and its 1.65″ case sits inside every case-length limit on the books, though every state writes its own rules and some have loosened them recently, so check the current regulations where you hunt. Winchester rates the cartridge for deer, hog, and black bear to 300 yards; the 215 gr Power-Point leaves a 20″ test barrel at 2,250 fps with 2,416 ft-lb, and from this 16″ expect roughly 100 to 125 fps less, which still carries more energy at 100 yards than a .30-30 or a 12-gauge slug.

How fast does 400 Legend leave a 16″ barrel?

Winchester publishes 2,400 fps and 2,430 ft-lb for the 190 gr Deer Season XP, 2,250 fps and 2,416 ft-lb for the 215 gr Power-Point, and 1,060 fps for the 300 gr Super Suppressed, all from a 20″ test barrel. A straight-wall cartridge burns its powder early, so a 16″ gives up roughly 100 to 125 fps on the supersonic loads, about 2,275 and 2,125 fps, and nothing on the subsonic. Sixteen inches is the length Faxon recommends as the best balance of velocity, accuracy, and handling for it.

400 Legend, 350 Legend, or .450 Bushmaster?

350 Legend if whitetail inside 200 yards is the whole job and you want the lightest rifle, the least recoil, and a barrel that runs a 5.56 bolt. .450 Bushmaster if you want the hardest-hitting straight-wall AR there is and do not mind the recoil. 400 Legend if you want the middle: by Winchester’s numbers, 25% more energy than 350 Legend with greater penetration, 20% less recoil than .450 Bushmaster, a 300-yard rating, and a factory subsonic for suppressed use. It costs a dedicated bolt (6.8 SPC) and magazines over the 350; the .450 costs a different bolt and ten more ounces of recoil. All three share the Big Bore extension, the .875″ or .750″ low-profile block, and Faxon’s straight-wall know-how.

Is the gas journal dimpled for set screws?

No. The journal ships round so any .875″ 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.