The AR-15 plays host to a few different calibers and cartridges. The military's 300 Blackout round became one of the most popular in recent years, and you can easily build or modify your AR-15 with a 300 Blackout kit. This dual-purpose round can be fired as a subsonic or supersonic cartridge, suppressed or un-suppressed, without having to modify your weapon (if you build it right). This guide breaks down 300 BLK in its entirety. We review ballistics and compare it to 5.56 NATO, we cover which parts are compatible with the AR-15's traditional 5.56/.223 parts, and what configuration - barrel, gas, twist rate, and buffer - is best for building a well-rounded, accurate, reliable gun. Let's get started.
What is 300 Blackout?
Officially called 300 AAC Blackout (designated 300 BLK by SAAMI standards), this round is a 7.62x35mm intermediate rifle cartridge. It was developed by Advanced Armament Corp (AAC) in 2009 to 2010 for the military's M4 rifle. Today, many shooters and gun owners build AR-15s chambered in 300 .BLK with ease, owing to the round's compatibility with the existing rifle platform. Director of Research at AAC, Robert Silvers summarized its development:
"We started development in 2009, but most of the work was done in 2010. A military customer wanted a way to be able to shoot .30-cal. bullets from an M4 platform while using normal bolts and magazines, and without losing the full 30-round capacity of standard magazines. They also wanted a source for ammunition made to their specs. We could not have just used .300-.221 or .300 Whisper because Remington is a SAAMI company, and will only load ammunition that is a SAAMI-standard cartridge. We had to take the .300-221 wildcat concept, determine the final specs for it, and submit it to SAAMI. We did that, and called it the 300 AAC Blackout (300 BLK)."
Approval & first impressions
The round was approved by SAAMI on January 17, 2011. Later that year, Army Staff Sergeant Daniel Horner used the round to win his fourth USPSA Multi-Gun National Championship. Modern shooters compare its ballistic performance to the common rifle round.
Ballistics, Charts & Data
When configured correctly, 300 BLK provides ballistic performance with "stopping power" similar to 7.62x39mm with a max distance comparable to 5.56 NATO. The ballistics data shown below was recorded using a 9-inch barrel (we'll explain optimal barrel length, twist rate, gas, and everything else next).
300 Blackout vs. 5.56 NATO
We recorded velocity, energy, wind drift, and bullet drop of both supersonic and subsonic 300 BLK against two of the most common 5.56 cartridges: The M855A1 (62-grain) standard ball round and MK-262 (77-grain), a popular match-grade round favored by SF operators overseas.
Velocity & Energy (9" Barrel, Supersonic)
We see that 5.56 NATO offers slightly greater muzzle velocity (around 400-500 FPS) than supersonic 300 Blackout, while offering decreased muzzle energy.
Bullet Drop & Wind Drift (9" Barrel, Supersonic)
Both 5.56 NATO loads also provide slightly greater resistance to wind drift and bullet drop when compared to supersonic 300 Blackout. While 5.56 NATO enjoys a slight edge in this case, the benefit of 300 Blackout is the ability to swap out supersonic for subsonic loads without re-configuring your rifle. Let's compare performance against those heavy sub loads.
Velocity & Energy (9" Barrel, Subsonic)
We can see right away that subsonic 300 Blackout out-shines 5.56 NATO, maintaining higher velocity and energy the moment it leaves the muzzle, all the way out to 700 yards.
Bullet Drop (9" Barrel, Subsonic)
The same holds true for bullet drop: Subsonic 300 Blackout outperforms 5.56 NATO by offering the same bullet drop from muzzle to 700 yards while affording over twice the kinetic energy.
Max Effective Range of 300 Blackout
By comparing to 5.56 NATO again, we can find the max effective range of 300 Blackout based on ballistic data. Note this is applicable for supersonic (125-grain) loads only. The U.S Military calculates the max effective range of 5.56 NATO to be 500 meters based on a 50% hit probability, which guarantees every other round will hit its target at the given distance. At 500 meters, 5.56's ballistic data looks like this:
- 100" of drop
- 41" of drift
- 291 ft. lbs. energy
300 Blackout's ballistic data is comparable at 440 to 700 meters
- 100" drop (440 meters)
- 41" drift (484 meters)
- 291 ft. lbs. energy (700 meters)
Officially, the U.S. Military has calculated supersonic (125-grain) 300 blackout's max effective range to be 440 meters. That's a loss of just 60 meters compared to 5.56 NATO.
300 BLK vs. 5.56/.223 Parts Compatibility
Building a rifle or pistol chambered in 300 Blackout is easy. Most of the parts you'll need are the same parts designed for 5.56 NATO or .223 Remington:
- Lower receiver
- Lower parts kit
- Gas block
- Gas tube
- Buffer tube
- Recoil spring
- Bolt carrier group
- Charging handle
- Upper receiver
The barrel and length of the gas system are the only things exclusive to 300 Blackout. All other AR-15 parts are compatible with this round. If you wanted to, you could simply swap the barrel and gas system length on an existing gun chambered in 5.56 or .223. This would convert it to fire 300 BLK without further modification required.
The Best Barrel Length
It's important to remember barrel length doesn't translate directly into more accuracy. The sole purpose of barrel length is to ensure pressure is built sufficiently behind the cartridge when fired, getting it up to its optimal velocity. The optimal barrel length on any weapon is reached when all powder in the case is burned and there's no more "fuel" to add to the combustion and gasses. At this point, increasing barrel length only provides marginal increases in velocity.
The optimal barrel length for 300 Blackout is 9 inches.
A nine-inch barrel guarantees all powder will burn in the cartridge and that the round fired will perform with similar ballistic performance to 5.56 NATO. If you didn't check the source above for its max effective range, take a second look: The U.S. Military and Advanced Armament Corp. calculated that increasing barrel length from 9" to 16" only increases max effective range up to 460 meters. That's a 20-meter increase for adding a whopping 6" of barrel.
Picking The Optimal Twist Rate
Firing light and heavy bullets without swapping out barrels means picking a twist rate that can handle both types. A 1:7 or 1:8 twist rate is optimal for 300 BLK. To pick between the two, simply consider whether you'll fire super or sub loads more often. If you're sticking with lighter supersonic loads for the most part, go for a 1:8 twist rate. If you plan on threading a suppressor on the muzzle to shoot mostly subsonic loads, go with the 1:7 twist rate.
Best Gas System Length
Even though subsonic 300 BLK bullets usually weigh twice as much as supersonic loads, the powder in the case itself has to be reduced to keep the round traveling under 1,125 feet per second. Less powder to burn means less energy makes its way through the gas block and tube to cycle the bolt.
To make up for this lack of energy, the shortest possible gas system must be used: You'll need a pistol-length gas system (4") for your 300 Blackout build. This is necessary to keep both subsonic and supersonic rounds cycling reliably without failures to feed or reset.
Optimal Buffer Weight for 300 BLK
This one's easy: Start with a standard H buffer weight (3.8 ounces). This is what Advanced Armament Corp. recommends by default and it's what most shooters say works best. An "H" buffer should reliably cycle subsonic and supersonic loads whether equipped with a suppressor or without, and felt recoil should be acceptable with either configuration. If your AR feels over-gassed or felt recoil is excessive, try an H2 (4.6-ounce) buffer instead. If you're getting failures to feed or unreliable operation, go lighter with a Carbine (3.0-ounce) buffer.
Frequently Asked Questions
Q: "Can I install a 300 blackout upper on an AR-15 lower?"
A: Yes. In fact, buying a pre-built 300 BLK upper and installing it on your AR-15's existing lower is the easiest way to get into shooting this unique round.
Q: "How can I modify my AR-15 to shoot 300 BLK?"
A: If you don't want to buy a full upper, just swap the barrel itself.
Q: "What magazines are compatible with 300 blackout?"
A: Any standard AR-15 magazine is compatible with 300 Blackout. Just be sure not to accidentally the wrong round in the wrong gun!
Q: "Can I shoot 300 Whisper, 300 Fireball, or .300-221 in my 300 Blackout upper?"
A: No. Differences in cartridge dimensions mean this would be unsafe, like firing 5.56 NATO in a .223 Remington chamber.
Q: "What's the best barrel length?"
A: A 9" barrel is optimal.
Q: "What's the best twist rate?"
A: Stick with a 1:7 or 1:8 twist rate. A 1:7 twist favors subsonic loads, a 1:8 twist favors supersonic loads.
Q: "What gas system should I use?"
A: Stick with a pistol-length gas system to be safe. It's the only one guaranteed to cycle subsonic loads while suppressed.