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?
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:
- It’s not as popular.
- 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.
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.
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
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.
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.
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?
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.