When it comes to AR-15s, your options for calibers are nearly limitless. Straight-wall cartridges for hunting, high-BC wildcats for long range, rebated-rim thumpers like .458 SOCOM or .50 Beowulf, pistol calibers, .410 shotshells–the sheer variety is mind-boggling.
But, despite the embarrassment of riches that is the AR-15 caliber catalog, most AR-15s manufactured today are chambered in one of two cartridges: the classic 5.56 NATO/.223 Remington, or the newer, slightly more exotic .300 Blackout.
Despite being the two most common calibers for AR-15s, the two cartridges really could not be more different. Where one excels, the other struggles, and vice versa.
Below, we’ll explore how.
The Basics: 5.56 NATO/.223 Remington
The .223 Remington and 5.56x45mm NATO cartridges share a great deal of their history. The .223 Remington came first, developed concurrently with the AR-15 as part of a military request for a replacement for the larger, more powerful rifles of yesteryear. 5.56 NATO came later, a slight modification of the original specifications in order to meet NATO objectives.
Today, the distinction is largely negligible. There are a few variances in the technical specifications of the two rounds, but they can in most cases be used interchangeably. The primary notable difference is in their chamber pressure, which leads us to the one instance in which they are most certainly not interchangeable–firearms chambered exclusively for .223 Remington.
While most modern .223 firearms are designed to accept either ammunition, some older weapons were designed solely for .223, and as such lack the reinforcement necessary to safely fire the higher-pressure 5.56 NATO.
In all other respects, the two rounds are more or less the same. Both fire a relatively lightweight projectile at fairly high velocities, creating a rather flat drop pattern compared to most similar intermediate cartridges. Bullets on the low end of the weight spectrum tend to be susceptible to wind drift though, necessitating the use of heavier-for-caliber projectiles for optimal long-range performance.
These cartridges are in a sense the proverbial jack of all trades, but master of none. They perform well across a wide range of applications, target types, and distances, but rarely match the performance of a purpose-built cartridge for any single application.
.223/5.56 rounds frequently rely on yawing or “tumbling” for terminal performance in live targets, such as game animals. Since a relatively high velocity is required to achieve this effect, effective range tends to decrease rapidly with barrel length. There are alternative solutions to this dichotomy, such as the use of mushrooming, hollowpoint, or fragmenting rounds, but each comes with its own tradeoffs.
.223/5.56 cartridges also suffer from limitations when it comes to suppression. Because the cartridges rely on velocity to achieve terminal effectiveness, subsonic ammo is impractical for the vast majority of applications, meaning that regardless of the effectiveness of the equipped suppressor, the round can never be quieter than a supersonic boom.
.223 Remington and 5.56 NATO also struggle at extremely short or extremely long ranges. At close range, the round is frequently, if anecdotally, reported to struggle to neutralize dangerous game before it can present a threat to the user. At long ranges, the lightweight bullets tend to buck wind poorly, shed velocity relatively quickly, and have limited terminal effectiveness.
The Basics: .300 AAC Blackout
Compared to the .223 Remington, which is nearly as old as the AR-15 itself, the .300 Blackout is a young cartridge. It was developed by AAC in the early 2000s to remedy some of the perceived shortcomings of the ubiquitous 5.56 NATO cartridge, including suppressed performance and short-range efficacy in combat.
The .300 Blackout drew heavy inspiration from older .30-caliber wildcats such as the .300 Whisper. However, unlike those rounds, the .300 Blackout is SAAMI-approved and standardized. Uniquely, a standard .223/5.56 AR-15 can be converted to .300 Blackout with only a barrel change, as the latter cartridge uses the same magazines and bolt carrier group as the former. This makes it a quick and easy upgrade to an AR-15 already chambered in 5.56.
.300 Blackout is heavily optimized for short-range performance. In supersonic loadings, its ballistics are not unlike those of common 7.62x39mm cartridges. Its terminal effectiveness is significant in the first few hundred yards, but its relatively low ballistic coefficient and heavier weight both create a fairly steep drop profile, with velocity bleeding off quickly at range.
Where the .300 Blackout shines is when used subsonically in concert with a suppressor. This setup allows some of the quietest sound signatures possible in an autoloading rifle while maintaining terminal effectiveness at short to medium distances.
Key Differences Between 5.56 NATO and .300 Blackout
The differences between 5.56 NATO and .300 Blackout are many, but the crux of the matter can be boiled down to this: 5.56 NATO is fast and light, while .300 Blackout is heavy and slow.
5.56 is more effective at extended ranges, particularly 300-400 yards. .300 Blackout, for its part, tends to be more effective at ranges closer than 50 yards, particularly with the right projectile. It can often offer a higher probability of a one-shot-stop on dangerous game like wild boar.
Unlike 5.56, .300 Blackout is not dependent on its velocity for effectiveness; instead, weight can be utilized to maintain muzzle energy and terminal energy while reducing velocity, allowing it to be effective even at subsonic speeds with the right ammunition. This makes it the perfect caliber for use with a silencer.
.300 Blackout also tends to offer better ballistics out of extremely short barrels, for a similar reason. Because 5.56 is heavily dependent on high velocities to be effective, short barrels tend to neuter its ballistics. A 5.56 round out of a barrel shorter than ten inches will result in an impressive fireball, but unimpressive muzzle velocity and poor terminal performance at all but the shortest distances.
All else being equal, .300 Blackout does generally have greater recoil than 5.56, but the difference is negligible for most applications. The larger discrepancy is in the matter of price; .300 Blackout is frequently significantly more expensive per round than 5.56, although the differential between the two is shrinking.
Choosing the Right Ammunition for Your Needs
In the end, mission always dictates gear, and the caliber you choose when building your best AR-15 is no exception. Your primary objectives should determine your choice of cartridge.
For the price-conscious, 5.56 will always be the round of choice. It’s a solid all-arounder, as capable at 50 yards as it is at 500. While it certainly won’t be hearing safe with a suppressor, few applications demand utter silence, and what it lacks in short-range performance it makes up for in versatility.
.300 Blackout, on the other hand, is really only required if you fit one of a few specific scenarios. Suppressed use, for example, heavily favors the .300 Blackout both for the overall sound profile and for terminal performance. As an entirely supersonic round, 5.56 simply cannot compare for suppression.
For ultra-short-range use, .300 Blackout again gets the nod, not only for its terminal ballistics and single-shot effectiveness but also because it can take advantage of shorter barrels without compromising its capability, making it ideal for highly compact firearms, with or without a suppressor.
Best Barrel Length for .300 Blackout
One of the primary advantages of .300 Blackout, as stated above, is its capability from very short barrels. The common consensus on 5.56 is that barrels shorter than 11.5″ are seldom worth the ballistic tradeoffs involved, and anything shorter than 10.3″ is simply a waste of powder. While we wouldn’t entirely agree, it’s unquestionable that .300 Blackout offers considerably superior ballistics in lengths shorter than this.
Due to several interconnected factors including powder burn rate, bullet weight, and more, .300 Blackout tends to offer a better compromise of ballistics from barrels shorter than 10 inches. Similar to 7.62x39mm, .300 Blackout achieves near-full powder burn at rather short barrel lengths. For most .300BLK loads, full-powder burn happens at around 8-9″, meaning that additional barrel length beyond that point will have diminishing, but still existent, returns.
Even with barrels shorter than this, .300 Blackout will still burn a considerable proportion of its powder, delivering ballistics closer to its peak potential. Barrels as short as 5.5″ are commonly used with great efficacy, whereas barrels this short are all but unheard of for 5.56 rifles.
While there is no one best barrel length for .300 Blackout, the cartridge does favor shorter barrels. Longer barrels, particularly those 16″ or greater, offer little in the way of enhanced ballistics for their increased size and weight. An 8-9″ barrel is often considered to be the ideal balance between velocity and size, while 5.5-6″ is frequently selected for highly-compact weapons that still require rifle-level velocity and terminal energy.
Conclusion
5.56x45mm NATO and .300 Blackout are very different rounds best suited to very different purposes. 5.56 is a jack of all trades, capable of performing a staggering variety of tasks with admiral effectiveness. .300 Blackout, on the other hand, is a master of only a few, but handles close-quarters target engagements and suppressed fire with nearly-unparalleled excellence.
Ultimately, the choice will largely be determined by your use case. Or, if you’re still unconvinced by either round, you may consider one of the other many, many options available; when it comes to AR-15s, the menu of available caliber choices is vast and varied.
For further reading, check out our companion article on .300 Blackout vs. 7.62x39mm.
