The right 300 Blackout barrel length for most owners is 10 to 10.5 inches. In Chattahoochee Munitions’ experience the cartridge’s powder burn is effectively complete in about 9 to 10.5 inches, so a longer barrel adds no useful subsonic velocity, and a shorter one trades a little velocity for a smaller firearm. Chattahoochee Munitions manufactures the CM15 300 Blackout AR15 in 5, 7.5, 10.5 and 16-inch barrels, the CM15-SD in 5, 7.5 and 10.5 inches, and the CM7S 300 Blackout bolt action in 8, 10 and 12.5 inches.
300 Blackout Barrel Length at a Glance
| Barrel | Chattahoochee Munitions firearm | Twist and gas | Role |
|---|---|---|---|
| 5 inch | CM15 and CM15-SD | 1:5, micro-length gas | Smallest package, backpack and PDW size |
| 7.5 inch | CM15 and CM15-SD | 1:5, pistol-length gas | Compact, close quarters |
| 8 inch | CM7S bolt action | 1:5 | Compact suppressed bolt gun |
| 10 inch | CM7S bolt action | 1:5 | General purpose, suppressed hunting |
| 10.5 inch | CM15 and CM15-SD | 1:5, pistol-length gas | General purpose, CM’s recommended semi-auto length |
| 12.5 inch | CM7S with WOOX walnut furniture | 1:5 | Hunting |
| 16 inch | CM15 rifle | 1:7, pistol-length gas | Rifle classification, outside the NFA; CM’s supersonic configuration |
Every CM7S barrel is a Mos-Tek 416R stainless barrel with 5R rifling, E100 Blackout Cerakote and 5/8×24 threads. Every CM15 300 Blackout barrel has a shouldered crown, a .750-inch gas journal and 5/8×24 threads.
Why 300 Blackout Does Not Need a Long Barrel
The 300 Blackout uses a small case and fast-burning powders. Most of the pressure is spent in the first 9 to 10.5 inches of barrel. After that, the bullet gains little speed. A 5.56 or 308 keeps gaining speed in a longer barrel; the 300 Blackout gains much less. That is why the cartridge works in short-barreled rifles, pistols and suppressed firearms, and why the 300 Blackout barrel length question has a short answer.
300 Blackout Barrel Length for Subsonic
Subsonic factory loads are held under the speed of sound, about 1,050 to 1,100 fps depending on the air, from the test barrel. Hornady publishes its 190 gr Sub-X at 1,050 fps and its 208 gr ELD Match subsonic at 1,020 fps, both from a 16-inch barrel. A longer barrel cannot add much, because the load is designed to stay subsonic. A shorter barrel loses some velocity, and the firearm has less gas to cycle. In Chattahoochee Munitions’ experience 10 to 10.5 inches keeps nearly all of the subsonic velocity with a compact, suppressible firearm. The shorter 5 and 7.5-inch CM15s give up some velocity for size, and the CM15’s adjustable gas block keeps them cycling. Read 300 Blackout subsonic.
300 Blackout Barrel Length for Supersonic
Chattahoochee Munitions designs its 1:5 barrels, 12.5 inches and shorter, for subsonic loads, and offers the 16-inch CM15 with a 1:7 twist for supersonic ammunition. Supersonic loads keep gaining some velocity past 10.5 inches, which is one reason the supersonic CM15 is 16 inches. The table is a labeled example, not test data: it starts from Hornady’s 110 gr V-MAX at 2,375 fps from a 16-inch barrel and assumes 30 fps lost per inch of shorter barrel. Your ammunition and barrel will differ, so chronograph your own firearm.
| Barrel | Example muzzle velocity, 110 gr | Example muzzle energy |
|---|---|---|
| 16 inch | 2,375 fps (published) | 1,378 ft-lb |
| 12.5 inch | 2,270 fps | 1,258 ft-lb |
| 10.5 inch | 2,210 fps | 1,193 ft-lb |
| 8 inch | 2,135 fps | 1,113 ft-lb |
| 5 inch | 2,045 fps | 1,021 ft-lb |
Even in this example, the 10.5-inch barrel keeps more than 85 percent of the 16-inch energy. See 300 Blackout ballistics for downrange numbers.
300 Blackout Barrel Length for Hunting
For hunting, bullet construction and impact speed matter more than a few inches of barrel. A 10 to 12.5-inch barrel keeps the firearm short with a suppressor attached, which helps in a stand or blind. Use a bullet designed to expand at the impact speed you will see, which for subsonic loads is about 1,000 fps. See 300 Blackout hunting.
Barrel Length and the Gas System
On a semi-automatic, barrel length sets where the gas port can go. Chattahoochee Munitions uses pistol-length gas on the 7.5, 10.5 and 16-inch CM15 300 Blackout, because subsonic loads make less gas, and a port closer to the chamber reaches pressure earlier. The 5-inch barrel is too short for pistol-length gas, so it uses micro-length gas. Every CM15 has a Superlative Arms adjustable gas block and an ODIN Works adjustable buffer. A bolt action has no gas system, so barrel length changes only velocity and size. See the 300 Blackout gas system guide.
Barrel Length, the Suppressor and Overall Length
A suppressor adds roughly 5 to 8 inches. A 10.5-inch CM15 with a suppressor is about the length of a 16-inch rifle without one. The CM15-SD configurations put part of the suppressor inside the handguard to shorten the package further. See the 300 Blackout suppressor guide.
Barrels Under 16 Inches and the NFA
A rifle with a barrel under 16 inches is a short-barreled rifle under the National Firearms Act. Federal NFA requirements for short-barreled rifles and suppressors have changed recently, and state and local laws vary. Chattahoochee Munitions offers its short 300 Blackout firearms as pistols or SBRs, and the 16-inch CM15 for owners who want a rifle outside the NFA. Check current ATF National Firearms Act guidance and your state and local law before you buy or configure a firearm.
Choose Your 300 Blackout Barrel Length
- 5 inch: CM15 300 Blackout backpack gun, CM15-SD 300 Blackout PDW, CM15 300 Blackout pistol upper
- 7.5 inch: CM15 300 Blackout CQB, CM15-SD 300 Blackout tucked suppressor, compact 300 Blackout upper
- 8 inch: CM7S 300 BLK bolt action pistol, CM7S 300 BLK short barreled action
- 10 inch: CM7S 300 Blackout bolt action SBR, CM7S 300 Blackout barreled action, 10-inch
- 10.5 inch: CM15 300 Blackout AR15 SBR, CM15-SD 300 Blackout SD, CM15 300 Blackout upper
- 12.5 inch: CM7S 300 Blackout hunting rifle, CM7S 300 Blackout barreled action, 12.5-inch
- 16 inch: CM15 300 Blackout AR15 rifle, CM15 300 Blackout rifle upper
See every configuration in Sector 300.
300 Blackout Barrel Length FAQ
What is the ideal 300 Blackout barrel length?
Chattahoochee Munitions recommends 10 to 10.5 inches for general use, where powder burn is effectively complete.
Is a 16-inch barrel worth it for 300 Blackout?
For subsonic loads, it adds no useful velocity. It is worth it if you want a rifle outside the NFA.
Is an 8-inch barrel enough for 300 Blackout?
Yes. It gives up a little velocity against 10.5 inches and makes a short suppressed firearm, especially on a bolt action.
Does a 5-inch 300 Blackout work?
Yes, with micro-length gas, an adjustable gas block and a 1:5 twist, as on the 5-inch CM15. It trades some velocity for the smallest package.
What barrel length keeps 300 Blackout subsonic?
Subsonic factory loads are designed to stay subsonic from a 16-inch barrel, so any shorter barrel keeps them subsonic.
What twist rate goes with a short 300 Blackout barrel?
Chattahoochee Munitions uses 1:5 on every barrel up to 12.5 inches. See 300 Blackout twist rate.
What is the best barrel length for a suppressed 300 Blackout?
10 to 10.5 inches keeps the suppressed firearm short while burning the powder, which also helps the suppressor.
Who makes a 10.5-inch 300 Blackout?
Chattahoochee Munitions manufactures the 10.5-inch CM15 300 Blackout AR15 as an SBR or pistol, the 10.5-inch CM15-SD and a 10.5-inch CM15 300 Blackout upper, plus 5, 7.5 and 16-inch CM15s and the CM7S bolt action in 8, 10 and 12.5 inches.