Many AR-15 owners begin to consider different cartridges after they’ve built or bought a few rifles in the standard 5.56x45mm/.223 Remington chambering. America’s love for .30-caliber cartridges is strong, and AR owners are not immune to the lure of a bigger bore. The .308 Winchester has been around for years. The 300 AAC Blackout is more than a decade old and has become quite popular, as well. Both cartridges can be chambered in AR-style rifles. However, there are some significant differences when pitting the 300 Blackout vs .308 Win. that can impact which you should choose for your next project.
History of Two Rounds: 300 Blackout vs .308 Win
The .308 is essentially a shortened version of the .30-06 Springfield cartridge used by the U.S. Military from 1906 through the late 1950s. The .308 is actually the commercial designation of the 7.62x51mm NATO cartridge. While there are a few technical differences between the two, most refer to them simply as “the .308.” If you’re familiar with historic battle rifles like the M14, HK G3, FN FAL or the M60 machine gun, then you’re familiar with the .308 cartridge they fire. The .308 is undoubtedly one of the most ubiquitous cartridges in America and around the world.
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The 300 BLK is a much newer round. However, the design inspiration behind it, a cartridge called the 300 Whisper, has been around for a long time. The 300 BLK standardized the performance and dimensions of the wildcat 300 Whisper for widespread use. It instantly exploded in popularity due to its ability to fit, feed and function in an AR-15. A standard 5.56mm AR-15 only needs a new barrel with a 300 BLK chamber and bore to fire this round. Even standard AR-15 magazines work with the 300 BLK.
Cartridge Size
Sharing the case head diameter of its parent 30-06 cartridge, the .308 is commonly used in just about every type of rifle. The one exception to that is the wildly popular AR-15.
Since the case head of the .308 measures 0.470 inches compared to the 0.378-inch diameter of the standard AR-15 chambering, the .308 isn’t a candidate for use in the AR-15. Not only is the case head too large, the 2.80-inch cartridge overall length is also far too long for use in the AR-15, which is limited to cartridges measuring approximately 2.26 inches long. If you’re running a bolt-action rifle then you can use either cartridge.
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The 300 BLK case is based on the .223 Remington case. This simple fact and all the benefits that come with it is responsible for much of the 300 BLK popularity. Since the 300 BLK shares a case head size with the .223, the standard bolt of the AR-15 can be used with either cartridge. The overall length of the 300 BLK is also the same as the .223, so standard AR-15 magazines can be used with either cartridge.
Performance Characteristics: 300 Blackout vs .308 Win
Once you’ve decided which rifle action you’ll be using, this is really where the differences between the two cartridges become apparent. The .308 is a medium-sized rifle cartridge with a powder charge somewhere around 36 to 46 grains with a typical 150-grain bullet. Common loadings include bullet weights between 150 and 180 grains. A 150-grain bullet leaves the muzzle of a 20-inch barrel at roughly 2,750 feet per second. Less common are loadings with bullet weights below 150 grains, down to about 110 grains, or weights above 180 grains. All of these bullets are intended to be fired at supersonic muzzle velocities. This is a powerful cartridge. The advantage goes to the .308 here.
The 300 BLK contains a much smaller powder charge. Most 110- to 150-grain bullets typically loaded into supersonic 300 BLK ammo receive around 20 grains of powder. A 110-grain bullet leaves the muzzle of a 16-inch barrel at around 2,250 feet per second. Compare those numbers with .308 ballistics and you can start to understand the differences between the two cartridges.
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300 Blackout: Slow & Low
The 300BLK has a trick up its sleeve, however. One of the biggest draws of the 300 BLK is its performance with subsonic ammunition. With the increased popularity of suppressors, the use of subsonic ammo has surged. Subsonic ammo doesn’t generate the “crack” of a bullet breaking the sound barrier. So, the overall noise level of firing is noticeably less than with supersonic ammo, especially when firing with a suppressor. Subsonic ammo may only have six to 10 grains of powder. Combine a significantly reduced powder charge with a bullet that never breaks the sound barrier, and a suppressed 300 BLK is significantly quieter than a suppressed .308. While the subsonic loadings generate much less velocity, they typically have bullets weighing more than 190 grains. Advantage goes to the 300 BLK here.
Making a Decision
Which one is right for you? Though the cartridges do have similarities, they are definitely on different ends of the performance spectrum. If you’re looking for a lightweight AR-15 then the 300 BLK is for you. There are a couple of proprietary .308 rifles that are almost the same size as a standard AR-15, but they’re specialized, more expensive and much heavier.
If you’re looking for the quietest suppressed performance, then the 300 BLK is for you. For those looking for a home-defense carbine, the .308 might be considered massive overkill. If you live on a large rural property and may need to defend beyond 300 yards, then the .308 is the better choice. The .308 is definitely the better choice of the two for long-range competitions.
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The .308 also offers significantly more power and range for hunting medium- to large game. However, those who already own an AR-15 can convert it to 300 BLK by simply removing the takedown pins and swapping the upper halves. That quickly makes your AR-15 a 300 Blackout. The best decision of all is to get both!
This post was updated on March 20, 2026.