.300 Blackout: A Tale of Two Subsonics

.300 blackout hornaday subsonic bullets in magazine

AAC or .300 blackout takes two essential forms: supersonic and subsonic. The latter will be the focus of this blog post.

The original cartridge, .300 Whisper, was developed by well-practiced wildcatter J.D. Jones to be a subsonic round that would fit, feed, fire and function in an AR-15 carbine.

Subsonic Blackout is a good fit with a suppressed gun because the additional back-pressure generated by a can is more welcome, and that’s something that can’t be said for other suppressed cartridges.

And, the reason for that is that getting reliable, the semi-auto subsonic function can be a trick.

I’ll do another entire article on that topic in these pages later on this year, but the reason is that the small amount of fast-burning propellant needed to launch a relatively heavy .308-caliber bullet doesn’t produce much gas to operate the system.

And, clearly, it’s not possible to run sub- and supersonic loads through the same gun without undue compromise. If it functions with subsonic, it will greatly “over-function” with higher-pressure supersonic.

(By the way, there’s no real difference in .300 Whisper and .300 Blackout. The main distinction is only a formality. Advanced Armament Corporation secured production-cartridge status, which means it got SAAMI-certified and called it .300 Blackout, aka AAC. Technically, Blackout can be a little higher pressure, which mostly only affects supersonic loadings.)

Why .300 Blackout?

hornaday aac rifle subsonic
The object of my attention and the motivation for this article: my “house gun,” a purpose-built .300 Blackout subsonic.

I got interested in subsonic Blackout after finishing a project for my last book, and that was building a specialized home defense gun.

I chose subsonic blackout because of its low “blast” (sound and flash) and also its impact performance at closer distances.

Impact performance, based on energy figures, puts most subsonic loadings at beyond routine .45 ACP loadings, so whether .300 Blackout is effective or not depends on what your opinion is of that round for a defensive application.

(By the way, the whole motivation for this wasn’t to do an article; it was to figure out what to load in my new gun! I chose one, and more in a short bit.)

The subsonic application is more specialized, meaning:

  1. It’s not as popular.
  2. It’s decidedly made from a different component mix.

There are not nearly as many options shopping for subsonic factory ammo, and I chose to evaluate two from the same manufacturer. These each also show the two essential approaches to subsonic blackout.

To be subsonic, muzzle velocity limit is just over 1000 feet per second (fps). Additionally, there has to be enough pressure to function the gun, but not exceed the velocity limit.

That combination of needs means that subsonic usually has a ballpark 200-grain bullet.

Hornady BLACK® .300 Blackout 208-Grain A-MAX®

Hornaday BLACK .300 blackout ammo 208-grain
Hornady BLACK® .300 Blackout 208-Grain A-MAX®

I have launched many, many A-MAX bullets at distant targets. Those, of course, were loaded to just as supersonic as I could get them!

This bullet is designed for competitive use, primarily, and follows the high-ballistic coefficient formula that’s proven to give less drop and drift at extended distances.

Hornady chose it for a subsonic blackout load because, as suggested, it’s the right weight to maximize energy at subsonic velocity, and at a functionally reliable pressure level.

Most factory subsonic loads follow this same formula, which, again, is to fit an existing 200+ grain bullet into this little case at a velocity of just over 1000 fps. The BLACK 208-grain subsonic is 1020 fps from a 16-inch barrel.

The A-MAX is not engineered for expansion, just good flight. The same can be said for most other bullets of a similar profile, like the Sierra 220-grain. MatchKing is popularly used by others.

Hornady .300 Blackout 190-Grain Sub-X®

Hornaday .300 Blackout Ammo 190 grains
Hornady .300 Blackout 190-Grain Sub-X®

This is different: “Subsonic eXpanding.” This bullet is designed to expand at subsonic velocity, and that’s made possible via a hollow cavity flanked by extra-long grooves on the jacket.

The cavity is capped and filled by a flat-nose polymer insert. Muzzle velocity is 1050 fps.

There are other effectively similar bullets out there (greater expansion at lower velocity), but the Sub-X is an example of a purpose-built bullet for a specialized application, and that’s usually going to work better.

Short course: It’s my choice, and it’s what’s in its magazine now.

How the Two Compare

hornaday subsonic 200-grain cartridges
The very “pointy” profile of the BLACK load’s A-Max is an asset to smooth feeding, no doubt, but the Sub-X fed and cycled crisp and clean.

I had no malfunctions with either while firing 40 rounds of each through my gun.

The 208-grain has more muzzle energy (480 foot-pounds, compared to 465). Not much difference, but it’s a difference.

Before running any numbers, I suspected that the significantly higher ballistic coefficient (BC) of the A-MAX (.648 compared to .437, both G1) might mean some advantage downrange, but it doesn’t.

Neither of these is moving fast enough to exploit any such. As a matter of fact, there’s a slight edge with the 190-grain.

Hornaday subsonic Nosler ammo
This represents an example of the “bigger is better” approach. It’s a 220-grain Nosler Ballistic Tip (round-nose) hunting bullet. That projectile has a proven record in the field (all Nosler hunting bullets do).

Now, on this topic: it’s a rainbow. I can’t speculate for others beyond my own beliefs, but a subsonic blackout has, at best, a very limited effective range.

It has an even more limited realistic range (meaning centering a shot on target). My intention with my build was to have a defensive carbine and, as such, I respect the circumstances in which it might be used.

That’s a max of 50 feet, not 50 yards. Zero one of these at 50 yards and there’s a solid seven inches (6.5 MOA) come up to get centered at 100. Going from 100 to 200 is dang near a yard.

Clearly, settle on a distance and settle the sight there, and get some practice with hold-over if you need to fire farther. 5.56 NATO is less than a half-inch from 50 to 100, and about three and a half inches 100 to 200.

I “accuracy tested” at 25 yards with these two loads and both clumped five-shot groups smaller than a golf ball. Way better than I do with a pistol! I’m beyond happy and confident with that.

Function Notes

Hornaday subsonic spring
There’s not a lot of pressure in a subsonic Blackout. I use a pistol-length gas tube and still need to cut coils to get reliable function. And! Your load choice might require unique tuning.

Again, this will all be in another article you can find on-site here in a few weeks, but I had to soften the buffer system to get 100-percent reliability with my subsonic.

All that means is that I cut a few coils from a standard spring, and that’s fit over a standard buffer. Don’t be bashful! Buffer springs are cheap if you make a mistake, but the function is priceless.

Another note is that, while it usually “works” running a standard 5.56/.223 Rem. magazine with .300 blackout, some rounds need another, and this is especially true with the subsonics.

The reason is that the larger diameter bullet effectively sits down lower atop the follower.

If the bullet profile is such that the bullet is contacting the follower then feeding issues will (not can) arise—the bullet noses upward. There are .300 blackout magazines, and you’ll find them on site here.

What About Others?

Hornaday subsonic Nosler ammo
This represents an example of the “bigger is better” approach. It’s a 220-grain Nosler Ballistic Tip (round-nose) hunting bullet. That projectile has a proven record in the field (all Nosler hunting bullets do).

There are subsonic rounds with bigger bullets, and those are viable. However! Make sure it’s a match for your chamber.

I’ve tried a couple that jammed into the lands when chambered.

Whether that’s truly a problem or not is a whole new article, but I’m not a fan.

(At the least chamber pressure will increase and the chambered round might not be able to be withdrawn).

What do you think of the .300 blackout? Are you a fan of subsonics? Let us know in the comments 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 (16)

  1. MJ asked: So other then flash and sound, what is the advantage of sub sonic? VS Disadvantages?

    The main advantage of a subsonic round is the ability to be whisper quiet with a good suppressor. I personally would not hunt with a ,300blk pistol/rifle at all. Supersonics are better for hogs and my deer rifle is better for everything else. .22 LR for varmint hunting. The .300blk subsonic was purpose-designed to hunt PEOPLE, namely terrorists and cartel being hunted by secret-squirrel special forces teams. You’re getting a big .30 cal round shot from a necked up .223/.556 cartridge and can use .556 magazines if need be (only thing you need is a .300blk barrel to convert most AR’s)

    Subsonic gives you stealth and quiet shooting up to 100yards with significantly more oomph (on close targets, think CQB) vs, a .556 round. It’s sort of the “happy middle ground” between an AR’s .556 and an AK’s 7.62×39 if you will. (.300 blk is analogous to a 7.62 x 35, in fact Magpul .300 blk mags actually imprint “7.62×35” on the top of their mags ). Again, Im talking INSIDE of 100yds.

    DISADVANTAGES – anything OVER 100 yards is not only harder to engage effectively but receiving a lot less kinetic energy due to the subsonic’s tendency to bleed off energy quickly. I hope that helps.

  2. Has anyone looked into the terminal ballistics of a subsonic 300 aac round like the Hornady 190 gr vs a 45+P hollow point on the flesh of 2 legged creatures? Would the larger .45 cross section yield a quicker stop to the threat? I cant find this discussed anywhere online, and i feel the question warrants significant consideration.

  3. Thank you for this information regarding this new caliber. I am interested in other related topics.

    Jack Alvarado

  4. I enjoy shooting 300. The ability to run 30cal through an AR is great since I can’t stand the AK. The People who say go with an AR10 are missing the point. A bolt gun is a better place for 308 and a bit much for social work. Not sure why your running a 16 inch barrel when 9 is all you need. It also allows room for a suppressor. Man up an pay for the SBR you will be glad you did. Thanks for the Good article.

  5. I’ve built a few. One set up for night shooting with night vision + FFP 1-6 tactical scope running Sig Sauer Elite Performance 208gr 1000f/sec subsonic. The other with a FFP 3-12X50 scope running the Hornady BLACK a-max 208gr. Both are zeroed at 50 yds. Both seem to run fine, so far. Have yet to fully test suppressed due to availability. I am primarily setting up for coyotes which have encroached into our area, hence the night vision and need for quietness. I am however more impressed with the Sig and it’s report.

  6. I built two .300 BLKs one pistol length and one carbine I hand load all my center fire ammunition so I opted to go supersonic for both. For me subsonic is a waste of time and money as performance is erratic not to mention the cost of factory ammo!!!

  7. I run a 230GR cast Lee bullet, powder coated in my 300BOs. Took a bit of experimenting to get it all running right, but it is very reliable now, and I can shoot 300BO for about the same price as 22LR.

  8. My home defense gun is an integrally suppressed (16” barrel) 300 blackout. I also clipped a few coils and it runs great with the 208 grain Hornady Amax. I hope I never have to use it, but I know it will function properly if I do. Great article. Thanks

  9. Good Article. I’ve just recently finished a .300 AAC build and have been reading everything I come across concerning its uses. So far I’ve only been using the supersonic loadings, but appreciate the input on the subsonic loads. Looks like I better order a second spring if I want to give them a try, and be prepared to trim it. I pulled the 1/2 dozen or so steel mags I had in my cans out and have stamped the baseplates .300AAC as a precaution against mixing the 2 calibers up. So all my 5.56 is in P-Mags, and the .300 wil be in steel or aluminum.

  10. I just run a PWS with auto gas setting. I can run subs or supers and no FTE, FTFs, etc…with or without a can. PWS makes an amazing system. 300 black in DI is way to problematic. I. Have a friend that has tweaked that setup to rn for a very specific SOF unit. Too risky for me tweaking gas tube ports or adding gas cut off keys, cutting buffer springs, no thanks. Buy a PWS! This is not a paid advertisement, they just work.

  11. I have been thinking about sub-Sonics for a while. I have built a Blackout with a 16” barrel and it functions flawlessly with supersonic ammo. My thinking is to go to a 300Blackout pistol- something in the 10-12” range and transition to subsonic. My concern is the possible need for a suppressor. I really do not want to go the NFA route, especially with a final cost for a suppressor in the >1.2K range ultimately. Will an adjustable gas block help with function in this kind of situation?

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