.300 Blackout vs. 5.56mm NATO - Which is Right for your AR-15?

The .300 Blackout and 5.56 NATO share the same AR-15 lower receiver, magazines, and bolt carrier group—but they are fundamentally different cartridges designed for different purposes. Understanding when each caliber excels, and where the other is the better choice, helps you decide whether to build or buy your next AR-15 in 5.56 or .300 Blackout.

What Is .300 Blackout?

The .300 AAC Blackout (7.62x35mm) was developed by Advanced Armament Corporation and Remington Defense for U.S. Special Operations Command, specifically to provide a suppressor-optimized cartridge that runs in a standard AR-15 lower with only a barrel change. It fires a .30 caliber bullet from the same case head as 5.56 NATO. In supersonic loads, .300 Blackout roughly matches 7.62x39 (AK-47) energy levels. In subsonic loads, it becomes the quietest practical AR-15 caliber when suppressed—the bullet travels below the speed of sound, eliminating the supersonic crack entirely.

5.56 NATO: Velocity and Range

The 5.56x45mm NATO cartridge is the standard AR-15 round—a high-velocity, light-recoil cartridge optimized for accuracy and terminal performance at distances from 50 to 500+ yards. From a 16″ barrel, 5.56 delivers approximately 3,100 fps with a 55-grain bullet, producing a flat trajectory and devastating fragmentation on impact from quality defensive loads like M193, M855, or bonded hollow points. It is the most widely available centerfire rifle cartridge in America and the round the AR-15 platform was designed around.

Where .300 Blackout Wins

Suppressed shooting is .300 Blackout’s strongest argument. Subsonic .300 Blackout loads (typically 220-grain bullets at 1,050 fps) produce hearing-safe sound levels through a quality suppressor—genuinely Hollywood-quiet compared to supersonic rifle cartridges. This makes .300 Blackout the top choice for suppressed home defense, hog hunting at night, and any application where sound signature is critical. At ranges inside 150 yards in the woods or brush, supersonic .300 Blackout also performs excellently on deer and hogs with expanding bullets, delivering .30 caliber terminal performance that 5.56 cannot match.

Where 5.56 Wins

Range, trajectory, and ammunition cost are 5.56’s advantages. Beyond 200 yards, 5.56’s higher velocity produces a flatter trajectory than .300 Blackout supersonic loads, and the energy advantage of the faster 5.56 bullet compensates for its smaller caliber at distance. 5.56 ammunition is also significantly less expensive than .300 Blackout, making high-volume training more affordable. For an all-purpose AR-15 that will be used for competition, general target shooting, home defense, and occasional hunting, 5.56 is the more versatile and economical choice.

The Bottom Line

Choose .300 Blackout if: you plan to suppress your AR-15, you primarily hunt at close range or at night, or you want maximum terminal performance inside 200 yards. Choose 5.56 if: you shoot at distances beyond 200 yards, you prioritize ammunition cost and availability, or you want a do-everything AR-15 without specializing for suppressor use. Because both calibers use the same lower receiver, many shooters own a single lower and swap between a 5.56 and .300 Blackout upper depending on the application. See also: .300 Blackout Ammo and 5.56/.223 Ammo.