FAXON DUTY SERIES GUNNER PROFILE 7.5" PISTOL-LENGTH 300 BLK AR15 BARREL

Rifle Barrels › Duty Series › 300 BLK › 7.5″ Gunner, Pistol-Length  |  Works with: 300 BLK BCG • .750″ gas block • pistol-length gas tube • 5/8x24 muzzle devices • .30 cal suppressors

Why 7.5″ Is the Right Length for a Suppressed 300 BLK

300 BLK was built to do two things a 5.56 cannot: run a heavy subsonic bullet quietly through a suppressor, and do it from an AR with nothing changed but the barrel. Both favor a short barrel. The cartridge burns a small charge of fast powder that is done working within about nine inches of bore, so a 7.5″ loses only 100 to 150 fps to a 10.5″ with supersonics and almost nothing with subsonics, which are limited by the speed of sound, not by barrel length. What you gain is an upper that, with a 6″ rail and a can, is about the length of a 16″ carbine with a bare muzzle, and a pistol that fits in a backpack with the suppressor off.

The Gunner profile on this barrel is built for a .30-caliber bore, not copied from the 5.56 version: a .980″ chamber journal, a .845″ body behind the gas block, and a .750″ journal, because a bigger bore leaves less wall and the steel has to stay where it stiffens the barrel. At 7.5″ that heavy contour still weighs only 0.87 lb. Pistol-length gas puts the port about 4.6″ from the breech with three inches of dwell past it, and Faxon reams it to .104″, sized for supersonics unsuppressed and subsonics with a suppressor. If you want more velocity margin for expanding supersonic loads and a longer sight radius, the 10.5″ Gunner is the same barrel three inches longer; if you will shoot only heavy subsonics, the Match Series 7.5″ in 1:5 twist is built for that. Faxon’s 300 Blackout barrel guide walks through length, twist, and setup for every use.

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, so the rifled dimensions Faxon air-gauges before treatment are the dimensions you get. 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 .3015″ 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 four grooves, right-hand, one turn in 8 inches, to a .3095″ 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 short section ahead of it is turned true to the same axis, and the 5/8x24 muzzle is threaded and crowned. Then the breech end: .845″ body, .980″ chamber journal, and the extension thread.
5. Chamber The 300 BLK chamber is cut and checked with a headspace gauge; chamber surfaces are held to a 20 Ra finish.
6. Extension, gas port, pin The M4 extension is torqued to 150 ft-lb, the .104″ gas port is drilled and reamed through the .750″ journal, the extension is pinned, and the barrel is headspaced with a 300 BLK go/no-go gauge.
7. Nitride and mark Salt-bath QPQ nitride inside and out, then the Faxon shield and order number are laser-engraved on the chamber journal and “.300 BLK 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 4 reviews on FaxonFirearms.com. Elsewhere: 5.0 stars across 3 reviews at Primary Arms, Faxon’s 300 BLK Gunner family averages 4.7 stars across 51 reviews at OpticsPlanet, and the line has a dozen 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 7.5″ Gunner 300 BLK FAQs

Is this a 300 BLK or 300 Whisper chamber, and what should I know about running it next to 5.56?

It is a SAAMI 300 AAC Blackout chamber, headspaced with a 300 BLK go/no-go gauge after the extension is installed. It will chamber and fire 300 Whisper ammunition, but load to 300 BLK data. The thing to know: a 300 BLK round will chamber in a 5.56 barrel and destroy the rifle when fired. If you own both, keep the magazines and the uppers visibly different and never mix them on the bench.

Why pistol-length gas on a 7.5″ 300 BLK?

Because 300 BLK is a low-pressure cartridge that burns a small charge of fast powder, and on a 7.5″ barrel there is no room for anything else: the port sits about 4.6″ from the breech, with roughly three inches of barrel past it, which is the dwell the cartridge needs to fill the system before the bullet leaves. The .104″ port on this barrel is sized so 110 to 125 gr supersonic loads cycle unsuppressed and subsonics cycle with a suppressor. This barrel needs a pistol-length gas tube.

What is the Gunner profile, and why is the 300 BLK version different from the 5.56?

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 .30-caliber bore leaves less wall than a .22, the 300 BLK Gunner keeps a .845″ body and a .750″ gas journal, where the 5.56 Gunner runs .750″ and .625″. At 7.5″ there is barely any barrel ahead of the block to taper, so it weighs 0.87 lb and takes the most common AR gas block.

What bullet weights does the 1:8 twist stabilize?

Both ends of the cartridge: 110 to 125 gr supersonic loads and 190 to 220 gr subsonics. 1:8 is the do-everything 300 BLK twist and the one most factory ammunition is loaded for. If you will run only heavy subsonics through a can, Faxon’s Match Series 7.5″ is offered in a 1:5 twist that stabilizes 220 gr and heavier from the shortest barrels; for a 7.5″ that shoots both, 1:8 is the right choice.

What gas block and gas tube does it take?

A .750″ gas block on the 1.9″ journal and a pistol-length gas tube. Faxon makes a .750″ adjustable and a .750″ fixed low-profile block, and a 304 stainless pistol-length gas tube. The gas port is reamed to .104″.

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 without adding thickness or changing the rifled dimensions that were air-gauged before treatment. 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 5/8x24.

Will it work with a suppressor?

Yes. The 5/8x24 muzzle takes any .30-cal direct-thread can or 5/8x24 mount, and this is the host the cartridge was designed around: 220 gr subsonics through a suppressed 7.5″ are about as quiet as a centerfire rifle gets, from a gun barely longer than the can. For a gun that runs suppressed most of the time, pair it with the Faxon .750″ adjustable gas block so you can bring the gas down.

Will it cycle subsonic ammunition?

Suppressed, yes: the extra back pressure from a can is what makes a 300 BLK run 190 to 220 gr subsonics reliably, and that is how the cartridge was designed to be shot. Unsuppressed subsonic cycling depends on the load, the buffer, and the spring, and no fixed gas port can be right for both a 2,200 fps supersonic and a 1,000 fps subsonic without a suppressor. This barrel’s .104″ port is sized for supersonics unsuppressed and subsonics suppressed. To run subs without a can, use a carbine buffer and a standard spring, pick a load with a fast powder, and pair the barrel with the Faxon .750″ adjustable gas block so you can open the gas for subs and close it back down for supers.

Is 7.5″ too short for 300 BLK? How fast does it leave the barrel?

No. 300 BLK finishes burning its powder in about nine inches of bore, so a 7.5″ gives up 100 to 150 fps to a 10.5″ with supersonics (expect roughly 2,100 to 2,200 fps with 110 gr loads) and essentially nothing with subsonics (about 1,000 fps with 220 gr, the same as any length). Where the extra inches of a 10.5″ or 16″ matter is the velocity margin for expanding supersonic bullets at distance; for a suppressed subsonic gun, 7.5″ is the length the cartridge was designed around. Faxon’s 300 Blackout barrel guide covers the trade-offs at every length.

Is 7.5″ the overall length, or the exposed length?

Overall: 7.5″ from the face of the barrel extension to the muzzle, the way barrel length is measured. About 1.3″ of that sits inside the upper receiver, so roughly 6.2″ is exposed ahead of the receiver face, and the muzzle threads take the last .625″ of it.

Can I put a 7.5″ 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.

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. 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.