What is 300 BLK for?  thumbnail image

What is 300 BLK for?

5D Tactical - 27th Jan 2023

While developing a rifle for the US infantry, Eugene Stoner envisioned a lightweight 20” barreled 5.56 rifle with the absence of a redundant forward assist. As the focus of intermediate caliber rifles evolved with a niche towards close quarter combat, the M4 barrel standards of 14.5” replaced the M16 barrel standard of 20”. With the addition of an IR illuminator, red dot (or holographic) sight, weapon mounted light, back up iron sights, foregrip, weapon weight becomes increasingly more apparent. For this reason 11.5” can be a desirable barrel length with AR pistols today. But with the reduction in barrel length comes consequently with a noticeable drop in projectile velocity especially for 5.56 NATO.

Designed from the ground up as a caliber for the AR platform, 300 AAC Blackout came to bridge the short barreled AR15’s 5.56 performance gap. 300 BLK is a round with a particular projectile to casing proportion. The bullet is bulkier with twice the grain of your typical 5.56 projectile, while the cartridge case is midget sized and scaled close to the same length of the projectile itself. This stubby specialty round is designed to excel within short engagement range through a 10.5” barrel.

What is 300 Blackout ACC supposed to do?

Within close range, 5.56 NATO has a tendency to punch through soft tissue rather than tumble and expand to properly transfer kinetic energy. This was a recurring problem for door-kicking special forces (Dutch MARSOF and unnamed units within US Special Operations Command and the United Kingdom’s armed forces) who have now adopted the use of 300 BLK. The requirement for a new round, which would become 300 Blackout ACC, to fix this drawback was:

  1. Deliver similar or greater force per pound of energy as a 7.62 x 39 round through an M4/AR15 platform
  2. Use .30 caliber projectiles
  3. Be compatible with the unmodified standard 30 round STANAG magazine
  4. Be compatible with the M4/M16/AR15 bolt
  5. Be subsonic and supersonic for suppression capability

We're not exactly sure if this was AAC’s own standards that they set for themselves during development or if these were entirely the requirements set by US Department of Defense for the contract bidders. Nevertheless, Advanced Armament Corporation took the CIP standard .300 Whisper, made by J.D. Jones and made it to SAAMI standards. The end result - 300 Blackout; passed all five criteria to win them the contract.

A suppressed, subsonic 300 BLK grain of 220 exceeds 2,000 ft/s when under 100 meters. But beyond that you will start to have questionable lethality. For 300 BLK, the simple solution is to switch out the magazine to full-powered supersonic 300 BLK rounds. This seamless switch doesn’t require gas block adjustments and guarantees someone running 300 Blackout will have sufficient firepower within 300 meters.

300 BLK vs 5.56 NATO and 7.62x39

Compared to its leaner 5.56 NATO sibling, 300 Blackout projectiles pack a lot more cake. A fatter bullet means a greater potential for transferring kinetic energy to the surface it strikes. The kinetic energy can be calculated with:


Kinetic Energy = (Bullet grain x Velocity2)/(14,000 x gravitational constant)

Gravitational Constant = 32.174 ft/s2

300 Blackout’s bullet opens up the means to deliver a great amount of energy, but there has to be the velocity behind it for a good performing round. AAC resizes their brass casing twice and anneal hardens the 300 BLK casewall to handle a hot load for a much higher pressure. With SAAMI standards, AAC is able to adjust the throat length longer than .300 Whisper and ensure compatible chamber specifications within a 300 BLK upper. These adjustments make 300 BLK an overachiever in close quarter combat even when using short barrels against its intermediate caliber competition, 7.62x39 and 5.56 NATO.

At a 7” barrel length, 300 BLK has a muzzle velocity of 2167 ft/s for a 110 grain projectile. This sends the projectile at its target with 1095 ft/lb force.

A stock Romanian Micro Draco with a barrel length of 7.75” holds a muzzle velocity of 1940 ft/s for a 123 grain projectile, yielding a force of 1027 ft/lb force

Lastly, a 7” AR15 barrel can push a 55 grain bullet out of the muzzle at 2410 ft/s, producing 709 ft/lb of force.

While 7.62x39 has similar ballistic performance as 300 BLK, this drastically changes when you start looking for subsonic rounds. Your options for subsonic 7.62x39 are very few as the AK typically doesn’t do well suppressed for an overgassed, non adjustable platform. 7.62x39 has a case taper that leads to feeding issues through an AR magazine design and introduces excessive wear on the bolt. It is precisely this reason that a .30 caliber round for the AR was designed from scratch.

For a close quarters M4/M16/AR15 platform, 300 BLK clearly reigns in the realm of quality and accuracy with suppression capability.

However, that isn’t to say that 5.56 NATO and 7.62x39 have been practically replaced. Ammunition costs and shooting past 300 meters will always be relevant, which are two features that 300 BLK does not excel within. Moreover, a fully loaded magazine of 300 BLK will be noticeably heavier than the same magazine loaded with 5.56 NATO.

Can you build 300 blackout in a 5.56 lower?

There is no such thing as a 300 Blackout lower. 300 BLK uses the same lower as any other AR15 chambered in 5.56 NATO and as such any AR15 lower will be a 300 BLK lower. Yes, you can — and only can build a 300 Blackout using a 5.56 lower.

Are there dedicated 300 BLK magazines?

Although 300 BLK rounds are cross compatible with standard 5.56 magazines, they are usually the first point of malfunction if any. For less than $16 per magazine, you can mitigate this feeding issue with a dedicated 300 Blackout magazine.

Best 300 Blackout Suppressors

What’s a suppressor focused caliber without a suppressor? Shooting subsonic rounds is essentially ensuring you have a projectile that has too much mass to exceed the sound barrier and make that crack of the whip sound. A suppressor stops the expanding gas from leaving the muzzle all at once by trapping it and dissipating it internally, as well as directing some of it back (back pressure) towards the action. This effect mitigates the other major noise factor to a gunshot, but it is also a critical feature to ensuring that your subsonic rounds are able to be cycled. People who shoot subsonic rounds (for whatever reason) will notice that their MCX Rattler has to be manually cycled when shooting those subsonic rounds unsuppressed.

Below we’ve gathered some worthy mentions of suppressor options to look into. These will help you protect your hearing and be considerate of anyone else that may be nearby wherever you’re shooting.

Why is the first shot with the suppressor the loudest?

The initial shot of the day using a suppressor won’t be much quieter than sitting in the front row of a rock concert. This is attributed to the reaction of the oxygen remaining inside the suppressor with the expanding hot gas that is trapped within the baffles. The first shot will burn this oxygen and the ensuing subsequent shots will feel much quieter.

Sandman S/L/K

Dead Air Silencers has a fantastic line of .30 caliber silencers with lifetime warranty. The full length Sandman L is optimized for performance out of the three suppressors — albeit without any compromise via weight saving measures, placing it at 21.8 oz (1.36 lbs) and 8.9” length. For a 7” barreled 300 Blackout AR this would put you into the familiar length of the standard NFA.

The Sandman S is shortened to 6.8” and reduces the weight down to 18.5 oz (1.16 lbs).

  • Multi-caliber. Suitable for 5.56 NATO, 300 BLK, 7.62x39. Rated for up to .300 Winchester Magnum
  • Quick detachable and can be removed with one hand
  • One of the quietest .30 caliber suppressors available currently

Sandman S

  • 6.8” length
  • 18.5 oz
  • $869.00

Sandman L

  • 8.9” length
  • 21.8 oz
  • $1050.00

Sandman K

Although there is an even shorter version called the Sandman K (5.4” and 12.8 oz), it sacrifices too much performance for reduced weight to be considered practical on a short barreled 300 BLK AR build.

Surefire SOCOM300-SPS

Made specifically for 300 Blackout ACC, Surefire’s SOCOM300-SPS is advertised as the “quietest Surefire suppressor designed”. While this all nearly looks externally the same as Surefire’s SOCOM762 RC2 suppressor, the Inconel steel baffles have been optimized for the back pressure to cycle subsonic 300 BLK. Its heat treated stainless steel outer construction (available in either black or flat desert earth) will hold up to the abuse and temperatures of the Surefire standard.

  • Designed specifically for 300 BLK carbines
  • Quietest Surefire suppressor available
  • Adapter mount compatible with Surefire prong flash holder and muzzle brake
  • 7.9” length
  • 18.6 oz
  • $1,169.00

Q Full Nelson

Manufactured entirely out of titanium, the Q Full Nelson promises resilience to heat and light weight for its size. The Full Nelson requires no muzzle device adapter and mounts directly onto your 300 BLK barrel via 5/8x24 threads. If quick detach is not a deal breaker for you, this is a great option.

  • Rated for up to .300 Winchester Magnum
  • 8.86" length
  • 16.6 oz
  • $849.00

Best 300 BLK Uppers

Ranked from the best uppers that money can buy to gold standards, we’ve assembled a list of a few 300 Blackout uppers that are worth the time and money.


As rare as most KAC 'unobtainiums' come in these times, the SR-30 is a 9.5” barreled beauty with a chrome lined 1:7 twist perfect for mounting onto the KAC SR-15 lower. If you can find it in stock currently then chances are that they’re price gouged to fend off anyone in the bottom 95 percentile of the US tax bracket.

Fortunately, KAC is pushing out production on the SR-30 again and the days of finding these bad boys readily available at less than 4 times the MSRP will come again.

Sig MCX Rattler

It’s the year of Sig Sauer. NGSW, XM17 Modular Handgun System, Romeo 8Ts replacing aged Eotechs; Sig Sauer has won massive contracts recently. They’ve been awarded the SOCOM Personal Defense Weapon Contract to supply US Special Forces Command with their MCX Rattler in both 5.56x45 and 300 Blackout. The 300 BLK Rattler variant comes in 23.5” overall length with a proprietary bolt carrier that eliminates the buffer tube, giving the Rattler a foldable brace/stock for a smaller profile. Its 5.5” cold forged barrel is lined with the ideal 300 BLK pistol 1:5 twist for subsonic rounds.


The familiar staple quality of Daniel Defense is available in 300 BLK with the DDM4 PDW. The PDW comes with the familiar DD furniture and a 7” cold hammer forged 7” barrel, altogether it stands at a length of 24.75”.

300 BLK Barrels

Already have an AR15 handguard, gas system, upper receiver in the length you want your 300 BLK AR to be in? Pick up a 300 BLK barrel to swap or add to finish your 300 BLK build.

Start your 300 BLK build today!

Already have thoughts about the perfect suppressible close quarters AR? Let us help you get started today. While our 80% gun build kits for 300 BLK are currently unavailable they are coming soon! Until then, we have products to help you build your next lower receiver