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 AR-15 for home defense that I also featured in 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.
Plus, I’m a believer in “bigger is better” respecting impact effectiveness of a bullet. That’s another debate for others to work though 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. This is really all about bullet design and 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 far after it meets a solid object. A 12 gauge slug, on the other hand, over-penetrates.
First, I’m always willing to risk boring knowledgeable readers with 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, without 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.
I have found that the best overall approach to subsonic function is to shorten gas system length. Run a pistol-length location gas port (4 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 that 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 gas expands behind the bullet traveling down the barrel’s bore, 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 is port pressure. Not technically the same as chamber pressure, but it’s from the same source.
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.
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, it has to run the subsonic, first! 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 7 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, not enough, and hotter and cooler.
One trick to get reliable subsonic function in a gun that’s been built around more ideal supersonic function is lightening 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 (varies with manufacturer), and working within this world of milliseconds, 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.
What’s your take on the .300 Blackout vs. 5.56 for home defence? Would you use a carbine- or pistol-length AR-15? Share your answers in the comment section.
The preceding is a specially-adapted excerpt from Glen’s newest book America’s Gun: The Practical AR-15. Visit for more articles and information.
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I ran a pistol length dedicated subsonic setup with suppressor. The biggest problem I found was the amount of unburnt powder building up in the chamber. This caused failures to chamber/go into battery and failures to extract from the chamber. After much experimentation I sold the 300 upper and went back to running a suppressed 9.25” gas piston 223 setup that runs like a freight train. As for 300 BLK supers, that’s what an AK is for. Oh, AK isn’t accurate? My Saiga 762 shoots .5 MOA with Golden Tiger. Buy a quality AK, use quality ammo.
The idea of building a 16” rifle optimized for subsonic .300 loads has never occurred to me. Why not just build a pistol with a 8”-10” barrel and install a brace? Much easier to use inside a house. Even with supersonic loads they are much more pleasant to shoot than a 5.56. I’m guessing they are also more likely to run reliably with subs/supers with less work. BCM advises that on their short barreled uppers you only have to switch between a Std. Carbine Buffer and a H3 when switching ammo types.
I have the 9″ BCM 300BLK and it reliably functions with subs or supers, suppressed or not, with a standard carbine weight buffer.
A 300 Blackout AR with 16” carbin length gas system will run using a Nemo Arms 300BLK light spring and a regular carbine buffer. This is another option if you don’t have a pistol length gas system in your barrel or do not want to cut a few coils off your carbine spring.
I run a 300 with a 16″ pistol gas system and and adjustable gas block. Have had no problems running clean with either supers or subs after I figured out the gas block. Just close it down 2 clicks for supers open it back up 2 clicks for subs. Yes I had that fiddle factor at first but now easy peasy
J D Jones says
The original 1992 300 Whisper AR by SSK ran clean and100% reliable with sub sonic and HV ammo due to the original two position original adjustable gas port setting. simply changing the name to Blackout and screwing up the gas system and most of the ammo I’ve tried doesn’t work well.
HW Stone says
While we go “what about” consider the 458 socom round for subsonics. Both the supersonic and subsonics like the same gas system, and while being hit by an angry motorcyclist going a hundred miles an hour is bad, being backed over by a fully loaded dump truck makes a rather deep impression, too.
And it is a piece of cake for suppression.
Can’t you just use subsonic ammo in an off the shelf rifle? Does that make it subsonic or not? What happens if you use subsonic ammo in super sonic rifle?
Aaron Kirkingburg says
Thanks, this was a good read! I literally just finalized my AR pistol that’s been sitting in my safe for almost 5 yrs waiting for an upper. I finally revisited the project and added the 8.5″ .300AAC upper specifically for use with sub-sonic ammunition. The BO was my choice of cartridge in a short home defense rifle for all the reasons you pointed out, and it was helpful to read some of your troubleshooting experience with operational issues and functionality. I feel I’ve went the right direction with everything so far!
Gabe Gallegos says
I think you are all full of it! I just brought a new Stag Arms blackout with a 9-inch barrel, and it runs both sonic and subsonic rounds like a bat out of hell, suppressed or not.