Subsonic Success: Getting Good Function From a .300 Blackout

subsonic .300 blackout gun front

Understatement: .300 Blackout (aka AAC) is a popular cartridge among AR-15 fans.

I like it especially in the shorter guns, and, around here at least, it’s looked on as an effective Whitetail cartridge choice.

Pretty much, it’s for those who want a bigger bullet in an AR-15 with a minimum of technical distractions (some call them “problems”).

One reason for its popularity is the supersonic/subsonic option. I built a specialty “home-defense” AR-15 for a recent book project, and that was my choice.

Reasons? Sure: it’s civil and effective.

Civil? I don’t know how many have fired a 5.56 AR-15 carbine inside a room, but it’s sensory overload.

In the dark, maybe just up out of bed, and then there’s a blinding fireball and an ear-splitting report, and it’s difficult to recover situational awareness, especially at my age, and even with my rail-mounted light.

Now, there are some very effective flash suppressors out there, but they don’t take a bit off the noise.

Subsonic Blackout has a radically milder blast and report than 5.56 or supersonic Blackout.

subsonic .300 blackout gun
Here’s my “house gun.” I trust it. Subsonic .300 Blackout is plenty powerful, in my estimation, and with radically better “shootability” than a higher-pressure carbine loading.

Plus, I’m a believer in “bigger is better” respecting impact effectiveness of a bullet. That’s another debate for others to work through in other articles, but it’s my story and I’m sticking to it.

Most subsonic blackout ammo uses a bullet in the 200-grain range, and, of course, .308 diameter.

Most factory .300 Blackout subsonic loads are a little more powerful than a routine .45 ACP handgun loading, if we’re going on (the admittedly incomplete) calculated energy figures.

So, if you think .45 ACP is a reliable choice for a defensive round, subsonic Blackout beats it. (Clearly, supersonic Blackout beats it soundly.)

Just a bit about the whole “defensive ‘rifle’ debate:” some say something like 5.56 is not a good choice for home defense.

I agree, but not for some reasons commonly given. There’s much said, unsubstantiated, about “over-penetration” of higher-velocity bullets. I don’t think that’s really a factor.

If anything, it’s the heavier bullets that are more likely to keep going. That really all has to do with bullet design/bullet engineering.

Any bullet that’s built to either fragment or readily expand (not the same things but about the same effect) isn’t going to get much farther after it meets a solid object. A 12-gauge slug “over-penetrates.”

Subsonic Architecture

subsonic .300 blackout gas system
A pistol-length gas system has been, for me, a key to getting fresh-off-the-workbench reliable function from subsonic Blackout. This is a 4-inch port location on a 16-inch barrel. Supersonic would totally overstress this system.

First, I’m always willing to risk boring knowledgeable readers with really basic information because it’s important to start at the start.

So, despite what I’ve heard from many theorizing, you really can’t run supersonic and subsonic loads through the same gun, with no modifications having been made to the gun.

There’s not enough gas in subsonic, or there’s too much gas in supersonic, for both to function through a system set up more ideally for one or the other.

subsonic .300 blackout gas ports
Two Blackout barrels: carbine port location on the stainless; pistol port location on the other. One for supersonic, the other for subsonic. It works!

I have found that the best overall approach to subsonic function is to shorten gas system length.

Run a pistol-length location gas port (four inches ahead of the chamber area) with a carbine-length (16-inch) barrel.

Done like that, the relatively tiny amount of fast-burning propellant behind that honking .30-caliber bullet gets put to work effectively because the pressure at the gas port is higher.

This is about the only time intentionally ramping up gas port pressure is ever welcome on an AR-15! I’ve written thousands of words about its evils and ways to lower it for other applications.

The pistol-length-port location requires the least amount of post-build tuning to get 100-percent reliability.

No room for a dissertation on gas system operation, but as the gas expands behind the bullet traveling down the barrel bore, an increasingly greater volume is available for the gas to occupy.

And, time is also ticking away with respect to the flaming consumption of the propellant.

The farther down the barrel the gas port is located, the lower the pressure will be by the time the bullet passes the port and the gas enters the port.

That’s “port pressure.” Not technically the same as “chamber pressure,” but it’s from the same source.

What’s more, subsonic ammo is a good deal lower pressure than supersonic ammo, which has a SAAMI max limit of 55,000 PSI (most is around 50-52,000); most subsonic loads are running 30-35,000.

And, yes, a supersonic blackout can have all the same extra-pressure-induced operation symptoms as a 5.56.

They’re not normally as overt, but they’re there—extra-quick bolt unlocking, excessive carrier velocity and on down that list.

Subsonic .300 blackout ammunition from Nosler
My magazines currently hold my most-trusted Nosler 220gr factory ammo. Good stuff.

If someone wants to build up an upper that could run both super-and subsonic, there’s going to be some parts mods involved before the cartridge switch.

But! First, it has to run the subsonic. A heavier buffer and spring, or an adjustable gas block, can provide the cushion the supersonic needs to avoid “over-function.”

The supersonic, though, needs a gas port positioned at seven inches forward, standard carbine location.

And! Speaking of adjustable gas blocks, don’t run one on a pistol-length system.

You might get away with it for a subsonic setup as described here, but since it’s so close to the chamber, the gas is at full fury and will wreck the valve apparatus in short order (flame-cutting).

I don’t like running a valve on anything but a rifle-length system.

Always (always) keep in mind that we’re operating in a world of fractional milliseconds defining “too much” and “not enough,” and “hotter” and “cooler.”

Supplemental Subsonic Tricks

Subsonic bullet .300 blackout
As you can see, it’s a big bullet.

One trick to get reliable subsonic function in a gun that’s been built around more ideal supersonic function is “lightening up” the back end of the system.

I’d suggest running a standard USGI-spec buffer and plain old standard variety carbine-length spring for subsonic.

Save the heavier and stouter parts for supersonic. I usually end up cutting 3-4 coils from the spring to add an edge of reliability to a subsonic.

I cut a couple even with my shorter gas system on the subsonic. Given an option, a little larger-diameter gas port adds more assurance that a subsonic will work through an otherwise supersonic setup.

Kind of like haircuts: they can take it off but can’t put it back.

In keeping with this motif, an “AR-15-style” bolt carrier works best with subsonic.

These are not as common now as they once were, but an AR-15 carrier doesn’t have as long of a full-profile section on its body as does the USGI-standard M16-format carrier (which is far and away the most common now).

An AR-15 carrier is about a half-ounce lighter (though it varies with each manufacturer), and, working within this world of milliseconds, that matters.

AR-15 subsonic carrier
Here’s a shrouded AR-15-profile carrier (left) next to a common and standard M16-profile. About 0.50-oz. weight difference. That matters!

On my build, I chose to accept the one-trick-pony approach. Right, it’s only good for subsonic, but it runs perfectly!

Running subsonic through my purpose-build supersonic blackout required changing a different buffer and a shortened spring and it runs, but it’s sluggish. A little edgy.

Have you had success getting good function from a .300 Blackout? Share your tips in the comment section below.


The preceding is a specially-adapted excerpt from Glen’s newest book, America’s Gun: The Practical AR15.

About the Author:

Glen Zediker

Glen Zediker is the owner of Zediker Publishing, which specializes in books and other publications focused primarily on AR-15s, handloading, and shooting skills. Since 1989, he has authored or co-authored 20 books.

He started shooting at age 5 and competing in NRA Smallbore rifle at age 8. He got his first AR-15 at age 15 and has now had 45 years of experience with that firearms platform. He’s worked professionally with some of the greatest shooters on the planet and leading industry professionals. And he does pretty well on his own! Glen holds a High Master classification in NRA High Power Rifle and first earned that using an AR-15 Service Rifle. He’s also competed in many other forms of competition, including USPSA, Steel Challenge, Silhouette Rifle and Pistol, Bullseye Pistol, ISSF Air Rifle, Practical Rifle and shotgun sports.

Since 1986 Glen has been a frequent and regular contributor to many publications, having had over 500 assigned articles published. See more at
The Mission of Cheaper Than Dirt!'s blog, The Shooter's Log, is to provide information—not opinions—to our customers and the shooting community. We want you, our readers, to be able to make informed decisions. The information provided here does not represent the views of Cheaper Than Dirt!

Comments (12)

  1. I assembled (hard to really say built) an Alpha Shooting Sports 10.5” .300 pistol upper with Nitride coated bag and with an Aero Precision lower, and have a Mystic X silencer. Fixed gas block and std buffer spring. I had it 6 months before I got the silencer and put 500-700 rounds through it.. This gun has shot subsonic and supersonic suppressed and unsuppressed flawlessly through 2500 rds min. I think carbine length systems and 16” barrels are part of the problem with .300 blk. This caliber has used up all the powder at about 9” of barrel, so it really affects carbine length systems imo. Lucky or good I’m not really sure which, but I’ll take it.

  2. When I first built a .300, I couldn’t get it to run reliably with bolt-over base failures, double feeds, etc. I eventually got an m-16 bolt carrier and heavy buffer. It cured the problem with supersonics, so I quit while I was ahead and only shoot those.

  3. Almost forgot about ammo. Forget about cut brass. Use factory loads and factory brass for reloading. The only issues I’ve had with my AAC barrel is THE with cut brass. Unless your 300 is just going to be a range toy, the cost savings isn’t worth the risk.

  4. Get an AAC barrel (pistol gas) and run M4 style BCG, buffer and spring and you are GTG with super or subs. That is what AAC designed the 300 blk to do and they know how to do it.

  5. Just recently purchased a pistol upper kit for thr build I’m working on. Now I understand better, what I’ll need to do if I want to run subsonic (the upper kit I purchased is set up for supersonic). Thank you for a clear and consisce desciption. For ease of use, I’d be better off building another just for using subsonic ammo.

  6. I have a 7.5″ 300 blk pistol with a weak spring & mil-spec buffer, completely closed adjustable gas block and it will only cycle 110gr V-Max rounds. Anything more and I get FTF,FTE. I now am thinking it could be dwell time. Thought about getting a 10.5″ barrel to see if it corrects the cycling issue. Thoughts or ideas?


  7. As far as reliability and changing the system from sub to supersonic on a pistols length ar pistol, I have had luck using the superlative arms non restrictive bleed off style gas block. I don’t know if it will have the short comings of “flame cutting” as listed above. I also have not put many thousands of rounds through it and I haven’t checked it for wear. I can say that it hasn’t seized up the way other restrictive style blocks have, and I feel that’s a good sign. I feel the concept by design puts less stress on the internal parts and I hope manufacturers continue developing in that direction in order to better refine how we adjust ar gas systems. I don’t like that I need a tool and I hope someone makes one that is finger adjustable or at least adjusted by using a round. I really like the FAL adjustable gas system. Bleed off/non restrictive, finger adjustable, battle proven, reliable.
    I hope this was helpful, cheers.

  8. I had ran the scenario of a suppressed subsonic .300 blackout against a suppressed 45/45+P (still subsonic). My research turned up the fact that without the ability of the high velocity of a supersonic .300 to transfer hydrostatic shock you will merely be punching .30 caliber rounds through your “target”. I see you have some rounds made to expand in this article. I had read those can cause more fed issue than FMJ in the AR platform. You seem to have found a sweet setup in this article that does not have any feed issues. So this brings me to my final points/questions. What is the diameter that your expanded subsonic .300 will achieve in the human anatomy vs that of the much wider 45(+P or even non +P for that matter)? Based on these points, for stopping a human target as quickly as possible, what would favor the subsonic .300 over the 45?

  9. I also discovered that some commercial .300 blackout pistols (like the CMMG Banshee 200) will not even cycle subsonic ammunition reliably without a suppressor attached. This was never listed in their product description, manuals, or even in their FAQs. They just expected people to “know” this.

    My pro shop at the range had no idea what was going wrong, they had me try different ammunition, including supersonic to make it work. The armor was going to charge me a lot of money to find the problem — until CMMG tech support told me. Btw: Supersonic ammo is also a really bad idea because of the VERY loud report and large muzzle flash.

    As everyone does know, it takes most of a year to get the ATF tax stamp approval for a suppressor, which means you’re stuck with a pistol that you can’t use, without damaging it, for a long time.

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