.300 Blackout — Subsonic Success Tips

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.

Subsonic Architecture

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.

Subsonic Tricks

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.