.300 Blackout Ammo

What is .300 Blackout?

Advanced Armament Corporation and Remington Defense developed .300 AAC Blackout in 2010 for a specific problem: U.S. special operations needed a suppressed AR-15 that hit harder than 5.56 at close range. The 9mm MP5SD was the previous standard for suppressed work. AAC’s answer was to neck up a 5.56 case to accept a .30-caliber bullet — same bolt, same magazine, same lower. Swap the barrel and you’re running a completely different cartridge.

SOCOM adopted it. SAAMI standardized it in 2011. The commercial market ran with it.

The core feature that makes .300 BLK unique: a single rifle runs both supersonic loads (110gr at ~2,350 fps) and subsonic loads (220gr at ~1,010 fps) — just change the ammo. No other common AR-15 cartridge does this. That flexibility is the whole point.

Supersonic vs. subsonic

This is the choice you make every time you load magazines.

Supersonic (110–150gr, 2,000–2,400 fps from 16”):

Supersonic .300 BLK is comparable to 7.62x39 in energy. Better than 5.56 for terminal performance at close range — the bullet is heavier and wider. Worse than 5.56 past 200 yards because the lighter 5.56 bullets carry BC better. Supersonic still makes a sonic crack through a suppressor, but it’s significantly quieter than unsuppressed.

Subsonic (190–220gr, 1,000–1,050 fps from any barrel length):

No sonic crack. Through a quality .30-caliber suppressor, subsonic .300 BLK drops to approximately 125–130 dB — near .22 LR levels. 150 yards is the practical limit before the round becomes aerodynamically unstable and drop becomes unmanageable. For suppressed use, this is a 0–100 yard cartridge.

Barrel length: it matters more here than in 5.56

Barrel length has a bigger effect on .300 BLK supersonic velocity than almost any common rifle cartridge. The powder is designed to burn fast in short barrels — which is by design for SBR and suppressor use — but you pay for it in velocity loss at longer barrels.

Subsonics don’t care about barrel length for velocity — they’re already slow. But gas port timing still matters for reliable cycling, which is why some very short barrels (under 8”) can have issues with subsonic loads.

Gas system cycling with subsonics

Most factory .300 BLK uppers cycle both supersonic and subsonic loads reliably. Problems show up in specific situations:

• No suppressor, subsonics: Some setups won’t cycle reliably without the added backpressure from a can. Test before depending on it.
• Adjustable gas block: Helps tune for both loads. Required if you’re having cycling issues.
• Heavy buffer: Many .300 BLK builds run a heavier buffer (H2 or H3) to improve subsonic cycling reliability.
• Very short barrels (under 8”): More problematic with subsonics. If you’re running a 7” pistol build, verify cycling with your specific setup.

The SIG Sauer MCX was specifically engineered for this — its short-stroke piston system cycles subsonics more reliably than a standard DI upper.

Should you even bother without a suppressor?

Honestly? Probably not.

Unsuppressed, supersonic .300 BLK is a mediocre cartridge. More expensive than 5.56, no steel-case ammo, shorter effective range, and similar energy envelope to 7.62x39 — which you can shoot all day for $0.45/round. The only advantage is running it in your existing AR-15 lower.

If you’re planning to get a suppressor and want the quietest possible centerfire rifle on an AR-15 platform, .300 BLK is the right answer. The $200 NFA stamp and 6–12 month wait is one-time. Once you’re in, the subsonic experience is genuinely different from anything else you can do with an AR.

Safety: the one rule you can’t ignore

Never chamber a .300 BLK round in a 5.56 barrel.

The .300 BLK round physically fits in a 5.56 chamber — it will headspace. When you pull the trigger, it destroys the barrel and can injure you or anyone nearby. This happens because reloaders and shooters mix ammo from different uppers.

Prevent it: mark every .300 BLK magazine with colored tape, mag couplers, or dedicated aftermarket mags. Do not mix .300 BLK and 5.56 mags in the same range bag.

Brand guide

Hornady Black 110gr V-MAX — The community standard for supersonic .300 BLK. Polymer-tipped V-MAX expands at .300 BLK velocities, which not all bullets do reliably. Consistent lot-to-lot performance. ~$0.75–0.95/rd.

Hornady Sub-X 190gr — Best subsonic with an expanding bullet. The Sub-X tip initiates expansion at subsonic velocities, which most hollow points won’t. Accurate, reliable cycling. ~$1.10–1.50/rd.

Barnes VOR-TX 110gr TAC-TX — All-copper construction, 100% weight retention, works in short barrels where cup-and-core bullets might not expand reliably. The choice for SBR home defense. ~$1.10–1.40/rd.

Federal Fusion MSR 150gr — Bonded hunting load, designed for semi-auto platforms. Good penetration for deer. ~$0.80–1.05/rd.

SIG Sauer 220gr OTM Subsonic — Reliable cycling in most platforms. OTM doesn’t expand but is accurate and consistent for subsonic range use. ~$0.90–1.20/rd.

Remington UMC 120gr OTM — The training standard. Cheaper than Hornady, reliable in most setups, not a hunting or defense load. ~$0.55–0.75/rd.

Fiocchi 125gr FMJ — Budget training, reliable. ~$0.55–0.70/rd.

.300 BLK vs 5.56 vs 7.62x39

Choose .300 BLK if: You’re running a suppressor and want the best subsonic capability on an AR-15 platform, or you need an SBR that hits harder than 5.56 at close range.

Choose 5.56 if: You want longer range, cheaper ammo, or you’re not running a suppressor.

Choose 7.62x39 if: You want AK-comparable energy without the AR-15 tax, and you’re fine with an AK platform.

Price guide (2025–2026)

No steel-case .300 BLK exists at any meaningful scale. Budget accordingly.

Firearms chambered in .300 Blackout

Semi-automatic (AR-15 based):

• Any AR-15 lower with a .300 BLK upper — the most common configuration
• SIG Sauer MCX — purpose-built for .300 BLK, short-stroke piston, better subsonic cycling
• CMMG Banshee 300 — pistol-format, 8” barrel
• PSA 300 AAC series — budget-friendly AR builds
• Springfield Armory SAINT in .300 BLK

Bolt-action:

• Ruger American Ranch .300 BLK — suppressor-friendly, threaded barrel, subsonic-capable
• Mossberg Patriot in .300 BLK
• Christensen Arms Ridgeline in .300 BLK

The bolt-action options make sense for hunters who want suppressed subsonic capability without the SBR paperwork — a 16”+ bolt gun in .300 BLK is standard Title I.