Short answer: a standard 5.56 bolt carrier group runs 300 Blackout: both cartridges have the same 0.378-inch case head. Your lower, buffer and usually your 5.56 magazines stay the same; only the barrel really changes. Mark your 300 Blackout magazines, because a 300 Blackout round can chamber in a 5.56 rifle, which is dangerous.
What changes and what stays
| Part | For 300 Blackout | Notes |
|---|---|---|
| Bolt carrier group | Same as 5.56 | Same 0.378 in bolt face |
| Barrel | 300 Blackout barrel | Usually pistol-length gas, 1:7 or 1:8 twist |
| Muzzle device | .30 caliber, usually 5/8x24 | Never a 5.56 device: the bore is too small |
| Magazines | 5.56 magazines usually work | Heavy subsonics feed best from 300 BLK magazines |
| Lower and buffer | Same | A heavier buffer can help short or suppressed rifles |
Magazines and the safety rule
Most 5.56 magazines feed supersonic 300 Blackout fine. Long, heavy subsonic bullets can feed better from magazines made for 300 Blackout. Whatever you use, keep 300 Blackout magazines clearly marked and separate from 5.56: a 300 Blackout round fits a 5.56 chamber, and firing it can destroy the rifle and injure you.
Parts that fit
Compare 300 Blackout barrels, 5/8x24 muzzle devices and bolt carrier groups, or start from our 300 Blackout starter build with live prices.
Frequently asked questions
Do I need a special bolt for 300 Blackout?
No. 300 Blackout uses the same bolt face as 5.56, so a standard AR-15 bolt carrier group works.
Can I use 5.56 magazines for 300 Blackout?
Usually, especially with supersonic ammo. Heavy subsonic rounds can feed better from 300 Blackout magazines.
Why mark 300 Blackout magazines?
A 300 Blackout round can chamber in a 5.56 barrel. Firing it is dangerous, so keep the two calibers clearly separated.
Do I need a different buffer for 300 Blackout?
Not usually. Short barrels and suppressors sometimes run smoother with a heavier buffer.
