What is 300 Blackout? It is a .30-caliber cartridge designed to run in a standard AR15 with only a barrel change, and to fire heavy subsonic bullets quietly through a suppressor from a short barrel. The case is a shortened .223 Remington case that holds a .308-inch bullet, so the AR15 keeps its bolt, lower receiver and magazines. Chattahoochee Munitions manufactures complete 300 Blackout firearms on the CM15 AR15 in 5, 7.5, 10.5 and 16-inch barrels, the CM15-SD with the suppressor under the handguard, CM15 300 Blackout uppers, and the CM7S bolt action in 8, 10 and 12.5 inches with matching barreled actions.
What Is 300 Blackout at a Glance
| Specification | 300 Blackout (300 BLK) |
|---|---|
| Bullet diameter | .308 inch |
| Parent case | .223 Remington, shortened |
| Case length | 1.368 inch |
| Maximum cartridge length | 2.260 inch (AR15 magazine length) |
| SAAMI maximum average pressure | 55,000 psi |
| Subsonic bullets | 190 to 245 gr at about 1,000 to 1,050 fps |
| Supersonic bullets | 110 to 150 gr at about 2,000 to 2,400 fps |
| AR15 parts that change | Barrel only |
| Standardized | SAAMI, 2011 |
Dimensions and pressure follow the ANSI/SAAMI centerfire rifle standard. Velocity ranges are taken from published factory loads.
What Is 300 Blackout Designed For?
The 300 Blackout solved one problem: put a .30-caliber bullet in an AR15 without new bolts or magazines, and make it work both subsonic and supersonic. Earlier .30-caliber AR15 cartridges needed their own bolt or magazine. Because the 300 Blackout keeps the .223 case head and the 2.260-inch length, a 5.56 bolt and a standard AR15 magazine feed it. Only the barrel changes.
What Is 300 Blackout Subsonic?
Chattahoochee Munitions treats the 300 Blackout as a subsonic-first cartridge. A 190 to 220 gr bullet at about 1,000 to 1,050 fps stays below the speed of sound, so there is no supersonic crack downrange. With a suppressor, what remains is muzzle blast the suppressor contains and the noise of the action. A heavy .30-caliber bullet still carries useful energy at that speed: Hornady publishes its 208 gr ELD Match subsonic at 1,020 fps and 480 ft-lb from a 16-inch barrel. Read the 300 Blackout subsonic guide for loads, cycling and suppressors.
What Is 300 Blackout Supersonic?
Supersonic loads push 110 to 150 gr bullets at about 2,000 to 2,400 fps, roughly the energy of a 7.62×39 load. Hornady lists its 110 gr V-MAX at 2,375 fps and 1,378 ft-lb from a 16-inch barrel. That makes the 300 Blackout a short-range hunting and defensive cartridge as well. Chattahoochee Munitions designs its 1:5 barrels for subsonic loads; for supersonic ammunition it offers the 16-inch CM15 with a 1:7 twist. Other cartridges do longer-range supersonic work better; see 300 Blackout vs 308 and 300 Blackout ballistics.
What Is 300 Blackout Used For?
- Suppressed shooting: subsonic loads through a suppressor in a short, handy firearm.
- Hunting hogs and deer at short range: with bullets designed to expand at the impact speed you will see. See 300 Blackout hunting.
- Home defense: a short-barreled AR15 or pistol with .30-caliber bullets and full powder burn in about 9 to 10.5 inches.
- Compact firearms: SBRs, pistols and backpack-size firearms, where the cartridge loses little from a short barrel.
For bear, choose a heavier cartridge. Chattahoochee Munitions recommends the 375 Raptor or the 458 SOCOM XL.
Barrel Length and Twist
The 300 Blackout burns its powder quickly. In Chattahoochee Munitions’ experience powder burn is effectively complete in about 9 to 10.5 inches, and a longer barrel adds no useful subsonic velocity. That is why CM recommends 10 to 10.5 inches as the general-purpose length. Heavy subsonic bullets are long, so CM uses a 1:5 twist on the CM15 5, 7.5 and 10.5-inch barrels and on every CM7S barrel. The 16-inch CM15 uses a 1:7 twist. See 300 Blackout barrel length and 300 Blackout twist rate.
300 Blackout and 5.56 Safety
A 300 Blackout cartridge can chamber in a 5.56 or 223 rifle, because the case head and length are the same. Fired in a 5.56 barrel, the .308 bullet is forced into a .224 bore and can destroy the firearm and injure the shooter. Mark 300 Blackout magazines, store the ammunition apart from 5.56 and check the headstamp before loading. Chattahoochee Munitions offers a CM15 300 Blackout magazine with a smoke-colored body for that reason.
Bolt Action or Semi-Automatic
The 300 Blackout works in both. A semi-automatic AR15 needs enough gas to cycle, and subsonic loads make less gas, so the gas system matters. Every CM15 uses a Superlative Arms adjustable gas block and an ODIN Works adjustable buffer. A bolt action has no gas system and no action noise when fired, which suits subsonic ammunition. See 300 Blackout bolt action vs semi-auto and the 300 Blackout gas system guide.
300 Blackout Firearms From Chattahoochee Munitions
- CM15 300 Blackout AR15 SBR, 10.5-inch
- CM15 300 Blackout CQB, 7.5-inch
- CM15 300 Blackout AR15, 5-inch
- CM15 300 Blackout AR15 rifle, 16-inch
- CM15-SD 300 Blackout SD, 10.5-inch
- CM15 300 Blackout upper, 10.5-inch
- CM7S 300 Blackout bolt action SBR, 10-inch
- CM7S 300 Blackout hunting rifle, 12.5-inch, with WOOX walnut furniture
- CM7S 300 Blackout barreled action, 10-inch
See every configuration in Sector 300.
What Is 300 Blackout FAQ
Is 300 Blackout the same as 300 BLK?
Yes. 300 BLK is the short name for the 300 Blackout.
Is 300 Blackout the same as 7.62×35?
Yes. 7.62x35mm describes the same cartridge by bullet diameter and case length. It is not the same as 7.62×39. See 300 Blackout vs 7.62×39.
What is 300 Blackout good for?
Suppressed subsonic shooting, short-range hunting of hogs and deer, home defense and compact firearms such as SBRs and pistols.
Is 300 Blackout subsonic?
It can be either. Subsonic loads use 190 to 245 gr bullets at about 1,000 to 1,050 fps. Supersonic loads use 110 to 150 gr bullets at about 2,000 to 2,400 fps.
Does 300 Blackout use a 5.56 bolt and magazine?
Yes. The AR15 keeps its bolt, lower and magazines. Only the barrel changes, though a dedicated, marked magazine is the safe practice.
Is 300 Blackout good for deer?
At short range with a bullet designed to expand at the impact speed, yes. Check your state’s rules. See 300 Blackout hunting.
What barrel length is right for 300 Blackout?
Chattahoochee Munitions recommends 10 to 10.5 inches for general use. Shorter barrels trade a little velocity for size, and a 16-inch barrel keeps the firearm outside the NFA but adds no useful subsonic velocity.
Do you need a suppressor for 300 Blackout?
No, but subsonic 300 Blackout is designed around one. See the 300 Blackout suppressor guide.
Who makes 300 Blackout rifles?
Chattahoochee Munitions manufactures 300 Blackout firearms: the CM15 AR15 in 5, 7.5, 10.5 and 16-inch barrels, the CM15-SD in 5, 7.5 and 10.5 inches, CM15 300 Blackout uppers, and the CM7S bolt action in 8, 10 and 12.5 inches with barreled actions.