The 300 Blackout, also written 300 BLK, is a rifle cartridge that suppressor owners favor because it has a standard subsonic load. Its official name is .300 AAC Blackout, and the ammunition industry’s standards body lists loads for it that run from light, fast bullets to heavy, slow ones. The heavy loads stay under the speed of sound, which is the reason suppressor owners talk about it so much.
This post covers what the cartridge is, what the official numbers say, how it compares with the .223 Remington, and one safety rule that every owner should know. It is general information and not a recommendation for a particular gun.
What the 300 Blackout is
SAAMI, the Sporting Arms and Ammunition Manufacturers’ Institute, writes voluntary standards for ammunition so that a cartridge from one maker fits a gun from another. Its centerfire rifle standard, ANSI/SAAMI Z299.4, lists the cartridge as “300 AAC Blackout” with the short name “300 Blk.”
SilencerCo says the 300 Blackout was designed as a subsonic load for close-quarters combat. In practice it is used for more than that, but the idea behind it explains the choices you see on the shelf. A subsonic load, in a suppressor maker’s words, is ammunition designed to travel below the speed of sound, which they put at 1,125 feet per second. Our post on subsonic ammo for suppressors explains what that line means.
The official loads and speeds
SAAMI’s standard lists five test loads for the cartridge. The speeds are measured 15 feet from the muzzle in a 16 inch test barrel.
| Bullet weight | Listed speed (feet per second) | Supersonic or subsonic |
|---|---|---|
| 90 grain | 2,500 | Supersonic |
| 115 grain | 2,270 | Supersonic |
| 120 grain | 2,100 | Supersonic |
| 123 grain | 2,280 | Supersonic |
| 220 grain | 1,020 | Subsonic |
The pattern is simple. The light bullets move fast, and the one heavy bullet moves slowly, at under half the speed of the lightest. That 220 grain load is the one that stays below the speed of sound.
A dealer’s guide from Silencer Shop describes typical subsonic .300 BLK loads as traveling roughly 900 to 1,000 feet per second with bullets of 200 grains or more. That lines up with SAAMI’s 220 grain figure. Specific boxes from specific makers differ, so read the label.
Those listed speeds belong to a 16 inch test barrel. Your rifle’s barrel may be a different length. SilencerCo says subsonic packaging often lists a recommended barrel length, so check the box against your own barrel before you buy.
How it compares with the .223 Remington
People often compare the 300 Blackout with the .223 Remington, a common rifle round. SAAMI’s standard covers both, which makes a fair side-by-side possible.
- Bullet weight and speed. The .223 Remington list runs from 35 to 77 grains at about 2,700 to 4,000 feet per second. The 300 Blackout list runs from 90 to 220 grains at about 1,000 to 2,500 feet per second. The 300 Blackout uses heavier bullets and, for most of its loads, lower speeds.
- Pressure. Both cartridges carry the same listed maximum average pressure in SAAMI’s transducer table: 55,000 pounds per square inch. Maximum average pressure is SAAMI’s limit for the average of a test sample.
- Test barrel. SAAMI sets most rifle test barrels at 24 inches. The 300 Blackout is one of a short list of exceptions and is tested in a 16 inch barrel.
These numbers do not say which round is better. They show how the two differ in bullet weight and speed. Of the two, only the 300 Blackout has a subsonic load, the 220 grain one, in the official tables.
Why suppressor owners like it
SilencerCo says that a suppressor paired with subsonic ammo lowers the sound more than a suppressor alone with standard ammo, and that using a caliber that was originally intended for subsonic use, such as the 300 Blackout, gives better results. A suppressor itself works on the noise of the gas leaving the barrel, and the maker says it cannot turn a fast bullet into a slow one. If you want subsonic results, you need a subsonic load.
That is why the 300 Blackout comes up in any conversation about a suppressed rifle. It is not the only way to build one. It is one cartridge with an official subsonic load.
A few cautions from the same maker apply. Some guns may struggle to cycle heavier bullets reliably, and factors like the barrel’s twist rate and the caliber can change how a subsonic load performs. We have no standard that says how every rifle will run every load. Testing a small box in your own gun is the only way to know.
The safety rule every owner should know
A 300 Blackout round can be loaded into a rifle chambered for the .223 Remington, and the result is dangerous. Windham Weaponry, a rifle maker, warns on its website that a 300 Blackout cartridge can be chambered in a 5.56 or .223 chamber “with catastrophic results.” The maker explains that the bullet is forced down into the smaller bore and the overpressure can make the gun fail. It reports that this led to returned rifles.
The maker’s advice is plain:
- Match your ammunition to the markings on the barrel.
- Check each round that goes into a magazine.
- Keep different types of ammunition separate on the bench.
If you own both a .223 and a 300 Blackout rifle, treat that advice as a habit, not a one-time tip. The barrel marking is the answer, and the ammunition box is the other half of the check.
What to ask before you choose
- Which cartridge is my rifle chambered for? Read the barrel marking, not the rifle’s look.
- Do I want the subsonic load, the fast load or both? Your answer changes the ammunition and may change how the rifle cycles.
- Is my suppressor rated for this cartridge? Ask the maker’s guidance for the cartridge, and ask us at the counter.
- Has the gun been tested with the load I want? Try a small box before a case.
- Is this ammunition interchangeable with 5.56 or .223? No. Windham Weaponry’s warning above exists because the two can be mixed up at the chamber, and the barrel marking is the check.
We can tell you what we know about the process of buying a suppressor and what to ask the maker. We are not able to say which rifle or load is right for you, so we will not guess.
For the cost and process of owning a suppressor, read our post on whether a suppressor is worth it.
What to do next
If you are thinking about a suppressor for a rifle, call or come by and ask. Our suppressors in Georgia page explains how buying one works here, and you can reach us through the contact page.
