The most common carbine round purchased by sports shooters is the 5.56x45mm NATO, which has remained the mainstay for the U.S. military rifleman for decades. While not identical in every capacity to its brother, the .223 Remington, the 5.56 remains part of the family of cartridges used widely throughout the U.S.

The 5.56 gained momentum amongst modern shooters alongside Armalite Rifle (AR) pattern weapons, whose popularity flooded the market in the late 90s. As the 2000s approached, shooters began using the AR-15 in sports and home defense as their firearm of choice.

The AR-15 pattern rifle was initially chambered in the .223 Remington when sold for commercial use. As the development of the M4 created a more compact carbine version with a shorter barrel, the product's popularity began to rise.

Today, most AR pattern rifles chamber the 5.56 NATO or the .223 Remington. But are these the best choices out there? When choosing the best cartridge for AR-style carbines, we aim to compare the 5.56x45mm NATO to the .300 AAC Blackout.

Introducing the .300 AAC Blackout

Despite its popularity, the 5.56 NATO round wasn't without its headaches. Feeding issues often occurred when integrating other rounds with the AR pattern, as there was no standard bolt compatibility.

The goal was to develop a cartridge that could resolve 5.56 performance issues while ensuring similar accessibility and capacity. This redesign led to the introduction of the .300 Advanced Armament Corp. (AAC) Blackout, or 300BLK.

With the .300 AAC Blackout, the firearm maintained many dominant features in the AR pattern family. However, it emphasizes changes that addressed the shortcomings of the 5.56 NATO round. Developed to launch 30-caliber projectiles from the AR platform, the .300 AAC Blackout offered the compatibility of a standard bolt with no reduction in magazine capacity.

Full-power, 123-grain ammunition matched the ballistics of the 7.62x39mm AK but had more energy than the 5.56mm M855. Due to the high efficiency of the .300 AAC Blackout, less powder is used than with the 5.56mm. The less powder used, the more comfortable the rifle is to shoot, even with a shorter barrel.

5.56 Performance in Carbines

While many tout the performance of the 5.56x45mm NATO round in rifles, the focus here is its use in carbines. Carbines are equipped with shorter barrels, thus requiring a different category of firearm classification.

AR pattern rifles adopted the shorter "carbine" format for military applications, requiring a more compact rifle variant without sacrificing rifle caliber performance from their cartridges. The M4, developed by Colt's Manufacturing, was the military's answer. Heading into the 21st Century, firearm owners quickly adopted carbines into their gun safes.

The M4 became widely used as a response to the issues faced by Rangers during the Battle of Mogadishu, becoming a focal point for several military branches during the wars in Iraq and Afghanistan. The U.S. Army rifleman had primarily transitioned to the M4, but the Marines and Navy began following suit during this era.

With the move to carbines in the military, noticeable performance deficiencies became more apparent. Special operations capable (SOC) groups began to assess other cartridges regarding improved weapon performance with suppressors and shorter barrel lengths.

The 5.56 NATO began to see trouble in military applications once riflemen noted a decreased ability to incapacitate an enemy combatant and losses in power and expansion upon impact (or fragmentation). The requirements of direct combat with compact carbines created the need for a higher-performant, close-range cartridge.

Growing Demands for the .300 AAC Blackout

The development of the .300 AAC Blackout was unique in that it focused on penetration and stopping power at short range, making a better alternative for the carbine-style weapons and close-quarter battles U.S. soldiers faced.

The caliber's creation ultimately resulted in a product that appealed to the Joint Special Operations Command (JSOC). It was compact and quiet when suppressed but had better accuracy and energy at a range comparable to the 9x19mm, often used in submachine guns.

While the round experienced limited use with special forces operators, use scenarios increased its visibility within the commercial market as a home defense option. The .300 AAC Blackout has grown in popularity and continues to be a cartridge of choice for shooters, as its ballistics make it better suited for narrow hallways and small rooms.

What Should You Chamber in Your Carbine: The 5.56x45mm NATO or the .300 AAC Blackout?

What's better, the 5.56x45mm NATO or the .300 AAC Blackout? While the question appears straightforward, the answer is more complex. Some shooters prefer variety when it comes to the guns they own; others find that they want to minimize the number of calibers they need. Regardless, the best caliber for you should match the intended job of your rifle, carbine, or pistol.

The .300 AAC Blackout has ballistic performance advantages, making it an ideal caliber for AR pattern carbines in home defense scenarios. It doesn't sacrifice capacity when using traditional AR magazines. As the carbine's system is nearly identical to the AR-15 that chambers the 5.56, its maintenance and handling remain similar.

Unfortunately, the round is more expensive than the typical 5.56 available in most firearm and sporting goods stores. The .300 AAC Blackout averages an additional .20 cents per round compared to the 5.56x45mm NATO.

Availability is also a potential issue, as manufacturers tend to focus on output for rounds that are in higher demand. As the 5.56 is most commonly purchased, the .300 AAC Blackout becomes a nearly impossible find when ammo is in short supply. So, plan accordingly if this is your choice for self-defense scenarios.

Ultimately, the cartridge is sometimes less diverse than we would hope, and we may need different options for purpose-built goals. As a purpose-built round for AR-style pattern rifles and carbines, the .300 AAC Blackout fits that description better than most.