Gun Digest

.300 AAC Blackout: The Whisper Turns Black

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During load development, the Loki Weapons System Patrol Rifle in 300 BLK was fitted with a Leupold Mark 4 6.5-20x ER/T variable to wring out its sub-MOA capabilities.
During load development, the Loki Weapons System Patrol Rifle in 300 BLK was fitted with a Leupold Mark 4 6.5-20x ER/T variable to wring out its sub-MOA capabilities.

The 300 AAC Blackout has both military and law enforcement applications and a growing legion of civilian fans to boot.

The Smith & Wesson Model M&P15 – 300 Whisper also chambers .300 BLK.

Advanced Armament Corp. (AAC) and Remington developed the 300 Blackout System to launch .30-caliber projectiles from the AR platform without reduction in magazine capacity, while maintaining compatibility with the AR-15’s standard bolt.

Remington received Sporting Arms and Ammunition Manufacturers Institute (SAAMI) acceptance of the new cartridge in June 2010. AAC officially designated it as the 300 AAC Blackout, or the 300 BLK. The metric designation is 7.62x35mm.

With subsonic cartridges, the 300 BLK produces a low sound signature when suppressed, and the development of 100- to 150-grain full-speed ammunition matches the ballistics of the 7.62x39mm AK. When fired from a 9-inch barrel, the 300 BLK produces more muzzle energy than a 5.56mm M855 round fired from a 16-inch rifle barrel.

The first priority of the 300 BLK was to utilize the existing inventory of 5.56mm NATO magazines while retaining their full capacity. Next, AAC wanted to create the optimal platform for sound and flash suppressed fire.

Another objective was to create compatible supersonic ammo that matches 7.62x39mm ballistics. The 300 BLK also provides the ability to penetrate barriers with high-mass projectiles. Providing all of these capabilities in a lightweight, durable, low recoiling package was what it took to meet the project’s objectives.

A standard M4 bolt is used for the new cartridge, and it matches or exceeds M4 endurance and durability. Compared with 5.56, a 7.62×39 or a 6.8 SPC, the BLK has reduced muzzle flash and “gasses” the shooter less.

When fired from a 9-inch barrel, the 300 BLK produces more devastating terminal effect than a 5.56, and on par with the 7.62×39 or 6.8 SPC.

Increasing Rate Of Fire

Installing a suppressor on a weapon typically increases the rate of fire. When comparing a suppressed to a non-suppressed 10-inch barreled 300 BLK AR to a 10-inch 5.56mm AR, the 300 BLK is a winner.

The average rate of fire for a 5.56 increases 34 percent, from an average rate of fire of 737 rounds per minute (RPM) to 985 RPM when suppressed. A supersonic load in the 300 BLK increases 18 percent, from 821 RPM to 970 RPM. Firing subsonic 300 BLK loads only upped the rate of fire 5 percent, from 746 RPM to 780 RPM.

Additional testing by AAC revealed that the 300 BLK has a point-of-impact shift with and without a suppressor of 1.3 MOA at 100 meters supersonic, and 2.4 MOA subsonic. On the endurance side of the equation, the mean number of rounds between stoppage was 738 rounds for the 5.56, and 640 rounds for the 300 BLK. Barrel life for a 5.56mm is typically about 9,000 rounds, whereas the 300 BLK has passed 30,000 rounds and continues to climb.

When comparing a 9-inch 300 BLK M4 PDW to an MP5-SD3, it really excels. The MP-5 weighs 7.9 pounds, and the 300 BLK PDW weighs 7 pounds when configured with a CTR stock, an H2 buffer and an AAC 762 SDN-6 suppressor with no magazine or sights. The compact length for the MP5 is 26.4 inches, and 31.7 inches with stock extended.

An M4 with stock collapsed and no supressor measures 25.75 inches, and 34.25 inches with a CTR stock extended and suppressor attached. Firepower is where the 300 BLK really excels. The MP-5 fires a 115-grain bullet at 900 fps, whereas the 300 BLK PDW firing a 220-grain bullet at 1,000 fps produces 129 percent more energy at the muzzle and even more down range.

“When people want to shoot suppressed, the MP5-SD3 has become obsolete because it can’t shoot through barriers; from armor, to wall board or windshields,” said Robert Silvers, director of research and development at AAC. “That’s why H&K came out with the MP-7A1, but you can’t make that gun quiet. You can keep an M-4 Blackout quiet, and it penetrates well. And, it makes a good home defense weapon, too. It has more energy than an M4 with a 16-inch barrel, and for people who want to hunt with their AR, it satisfies that, too. The Blackout is extremely versatile and can handle bullets from 110 grains to 240 grains.”

Shooting the 300 BLK

The basic difference between the 300 BLK chamber and the .300 Whisper or .300 Fireball is the length of the throat. The 300 BLK is slightly longer. One of the quirks of the .300 Whisper is the variation in brass thickness among the many makes of 5.56mm ammunition. The 300 BLK’s longer throat mitigates the issue.

When I asked Silvers about the longer throat in the 300 BLK, he said, “The people that don’t like the longer throat seem to always bring that up. We get good results from a test barrel. We shot 10 five-round groups that came in under .85-inch.”

The 300 BLK is a short-range round. When it is zeroed at 100 yards, the 300 BLK, loaded with a sub-sonic 220-grain Sierra Match King, drops nearly three feet when it passes the 200-yard mark. The same zero with a 125-grain Nosler Ballistic Tip bullet traveling at 2,200 fps will drop approximately 7.3 inches at 200 yards.

Since Remington hadn’t shipped any 300 BLK ammo prior to my initial testing, I spent some time forming Lake City mil-surp brass with a Redding 300/221 Fireball trim die. I scrounged some .221 Remington Fireball brass, too, for reforming. When I ran across Brad’s Warehouse ( as a source for 300 BLK cases, I jumped on 1,000.

Just as this article was about wrapped up, AAC shipped me an excellent supply of primed Remington 300 BLK cases, too.

My initial loads were educated guesses to determine velocities. Serious accuracy loading would come later. Cases were primed with CCI 400, Federal Small Rifle Benchrest or Winchester Small Rifle primers. I loaded H110, Accurate 1680 or Reloader No. 7 powder for initial testing. An AAC .30-caliber Cyclone suppressor was used for all testing.

Testing AR Platforms

To learn more about the 300 BLK, I tested three AR platforms. I got in touch with AAC and received one of the first barrels available, along with a low-profile carbine-length gas block and gas tube. To build a complete upper receiver suitable for testing, I had Anderson Manufacturing, from Hebron, Ky., mate the AAC barrel with one of their quality upper receivers and hand guards.

AAC’s 300 Blackout AR-15 barrels are 4159 chrome-molybdenum vanadium steel with a 1:8 twist. They utilize a standard M4 barrel extension, and have a high-reliability self-loading specific chamber. The chamber dimensions are slightly larger than found on bolt guns to increase reliability.

Their barrels are treated with proprietary nitride surface treatment on the barrel and extension assembly to give up to a 60 percent longer barrel life than chrome plating the barrel. The treatment improves corrosion resistance, reduces friction and improves accuracy issues, like hard chroming the barrel and creating coating thickness variations. The muzzle is threaded 5/8 24 TPI and comes with a thread protector.

At the range, I attached the AAC/Anderson Manufacturing upper to a Bushmaster lower fitted with an AR Gold trigger, and was ready to send rounds down range. Using a Leupold 6.5-20x Mark 4 scope, I shot several five-shot subsonic 100-yard groups.

The average for 12 groups fired was a hair under 2 inches, with two groups measuring sub MOA. Hornady’s 208-grain AMax, Sierra’s 210- and 220-grain Match Kings all shoot well in subsonic loads.

Another carbine tested was a Loki Weapons System Patrol Rifle. In short, this rig is comprised of Loki’s billet upper and lower, an Ergo F93 Pro adjustable stock and Loki’s forend and adjustable gas block set at the carbine position on a 16-inch Satern barrel.

I fired a hodge-podge of reloads trying to find the performers, just as I had done with other rigs. I even tried two Corbon .300 Whisper factory loads. No matter what I threw at it, this rig performed well.

The average of all five-shot groups measured 1.7 MOA. I shot one .753-inch group with a 220-grain Sierra Match King on top of 10.8 grains of A1680. Corbon’s 220-grain SMK subsonic load also shot a .783-inch group. Corbon’s 125-grain .300 Whisper load averaged less than MOA, with the smallest .811-inch.

Today, Hornady has joined the .300 BLK fray with their .300 Whisper loads. These function well in weapons marked 300 BLK or .300 Whisper.

Although I’ve always enjoyed shooting the .300 Whisper, or the .300 Fireball, the new 300 BLK designation is sure to shine some new light on this interesting cartridge. It’s certainly has attracted significant consumer acceptance.

This article is excerpted from the January 13, 2014 issue of Gun Digest the Magazine.

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