ArmaLite-pattern rifles, carbines, and pistols are chambered in various cartridges for different applications, both general and specialized. That’s one of the advantages of the AR-15 platform — modularity.
By simply changing the upper receiver assembly or just the barrel, you can change the caliber of your rifle, delivering improved accuracy or ballistic performance to tailor your weapon to the task at hand.
Two of the most popular calibers in modern semi-automatic rifles are 5.56×45mm NATO, the standard AR-15 cartridge, and .300 AAC Blackout, a recent addition to the list of defensive options.
Intermediate Calibers
One of the defining characteristics of a rifle is that it fires a cartridge intermediate in power between a submachine gun and rifle cartridges. Submachine gun cartridges are the same as those fired from handguns — e.g., 9×19mm Parabellum and .45 ACP. In the West, military handgun cartridges typically use bullets weighing between 124 and 230 grains propelled to muzzle velocities between 850 and 1,250 feet per second.
Full-power rifle cartridges include the .30-06 Springfield, .308 Winchester/7.62×51mm NATO, and 7.62×54mmR. These rounds are typically loaded with bullets weighing 147–175 grains at 2,500–2,900 feet per second.
Intermediate cartridges, such as the 5.56mm NATO, 5.45 and 7.62×39mm Soviet, and .300 Blackout, propel bullets typically weighing between 53 and 125 grains at muzzle velocities between 2,350 and 3,250 feet per second. As these rounds generally are shorter than their battle-rifle counterparts, they allow for correspondingly shorter actions and relatively compact weapons.
Lower on the power spectrum, they also increase the controllability of fully automatic or burst fire while still inflicting sufficiently traumatic wounds to incapacitate enemy soldiers at several hundred meters.
In modern semi-automatic rifles, intermediate ammunition provides the private citizen with a versatile, maneuverable, and effective weapon for self-defense, hunting, wilderness survival, and other applications. While fully automatic fire is not generally available to private citizens, these calibers are also more controllable during rapid semi-automatic fire. This can be beneficial in both defensive and competitive shooting scenarios.
5.56mm NATO
The 5.56mm NATO is the standard AR-15 cartridge. Along with its predecessor and the .223 Remington, the 5.56mm NATO is one of the most popular centerfire rifle cartridges in the United States.
As a brief history, in 1962, Remington submitted the specifications for the .223 Remington cartridge to SAAMI (the Sporting Arms and Ammunition Manufacturers’ Institute). In September of the following year, the United States Army type-classified the .223 Remington cartridge as the “Cartridge, 5.56mm Ball, M193.” This cartridge was loaded with a 55-grain, .224-caliber bullet. In 1964, Remington introduced the .223 Remington cartridge to the civilian market, coinciding with Colt’s commercialization of the AR-15.
In 1980, NATO standardized the 5.56×45mm cartridge, selecting FN’s SS109 load. The SS109, designated the M855 in the United States, uses the heavier 62-grain bullet with a green tip. This bullet contains a 10-grain, .182-caliber hardened steel insert, which increases its penetrating capability.
The 5.56mm USGI and NATO cartridges and the commercial .223 Remington are the standard chamberings for the AR-15 and M16 series of rifles. While the use of 5.56mm ammunition in a rifle with a .223 chamber is considered unsafe due to differences in chamber design and pressure curves, the reverse is acceptable. If you have a rifle with a .223 Wylde hybrid chamber, you can fire both cartridges safely and accurately.
.300 Blackout
A relative newcomer to the ammunition scene, Advanced Armament Corporation developed the .300 Blackout cartridge between 2009 and 2011. AAC designed this round to meet the requirement for a .30-caliber intermediate-power cartridge that could be fired in a 5.56mm AR-15/M4-pattern weapon without changing the bolt carrier group or magazine. The only required change is the barrel.
Furthermore, when using the standard military-specification 30-round USGI/STANAG magazine, there should be no reduction in capacity, as in some other caliber conversions (e.g., 6.8mm SPC).
In 2011, AAC submitted the specifications of a wildcat cartridge that it developed using .221 Fireball cartridge brass to SAAMI.
The .300 Blackout is available in both subsonic and supersonic loadings. When subsonic, the .300 Blackout uses a relatively heavy bullet, typically between 190 and 220 grains, to lower the muzzle velocity. When loaded to supersonic velocities, the .300 Blackout uses bullets ranging from 110 to 150 grains.
One of the reasons for the .300 Blackout’s popularity is its optimization for use with sound suppressors.
The .300 Blackout cartridge, which uses either the .223 Remington or .221 Fireball cartridge case, can use the existing .223-caliber/5.56mm breech face. As a result, the only change to the upper receiver assembly necessary is the barrel. Depending on the type of ammunition you fire, you may also need to change the buffer.
If you use .300 Blackout ammunition in the vicinity of 5.56mm rifles, it’s imperative that you pay careful attention to the rifles in which you’re loading these cartridges. A .300 Blackout cartridge can chamber in a 5.56mm rifle, headspacing on the bullet rather than the cartridge case shoulder. An unsuspecting shooter, experiencing a failure to chamber, may also force the bolt home using the forward assist. Firing a .300 Blackout cartridge in a .223 chamber can cause a catastrophic failure.
Advantages of the 5.56mm NATO
The 5.56mm round has several advantages in comparison to the .300 Blackout. These include the following:
● Lightweight ammunition
The 5.56mm cartridge, using a lighter bullet, allows you to carry more rounds of ammunition per unit of weight. This was one of the reasons for its adoption in comparison with the 7.62×51mm NATO cartridge.
● Accuracy
The .300 Blackout cartridge is capable of a high degree of accuracy. However, the 5.56mm cartridge in rifles and carbines achieves a considerably higher muzzle velocity than supersonic .300 BLK, thereby producing a more flat trajectory. When using match-grade target ammunition, a 5.56mm rifle is more suitable for precision shooting.
● Less recoil
Using bullets weighing between 55 and 77 grains, the 5.56mm generates a lower recoil impulse, all else being equal, than the .300 Blackout; about one-third less. However, as both cartridges are in the intermediate power class, this may only be a concern when using notably lightweight weapons or during competitive matches. If necessary, you can use a compensator or muzzle brake to reduce the muzzle climb, recoil, or both.
● Reduced penetration
This can be both a positive and a negative, depending on the application. In the context of home defense, wherein you’re shooting at a target inside a house, the 5.56mm round exhibits less interior-wall penetration. Lightweight, high-velocity bullets are more likely to tumble or fragment when they strike drywall and plywood. Furthermore, many .223-caliber jacketed soft-point, hollow-point, and frangible bullets are available for this purpose.
● Availability and cost
One of the primary advantages of the 5.56mm cartridge is availability compared with other AR-15-compatible chamberings. NATO standardization and the widespread use of the 5.56mm cartridge in light machine guns and rifles ensure that industry will always endeavor to supply ammunition to the market.
In addition, chamber gauges, reloading supplies, and rifles remain in high demand. As for cost, even during periods of scarcity, 5.56mm ammunition tends to be less expensive to purchase than .300 Blackout.
Advantages of the .300 Blackout
A more specialized round, the .300 Blackout has its share of benefits. These include:
● Increased penetration
Outside of a home-defense context, where interior-wall penetration is a liability, the .300 Blackout exhibits superior barrier penetration compared with the 5.56mm M855 cartridge due to increased projectile mass and kinetic energy. If you need to penetrate cover, such as sheet metal or auto glass, to hit an assailant, the .300 Blackout packs more of a punch.
In a hunting context, the increased penetration of the .300 Blackout cartridge also renders it a more suitable choice for hunting deer and hogs.
● Power
The .300 Blackout, when supersonic, can outperform the 7.62×39mm Russian, retaining kinetic energy more efficiently at longer distances. The round is also less affected by barrel length regarding terminal performance than the 5.56mm cartridge. While the 5.56mm cartridge, when loaded with full metal jacket ammunition, can inflict highly traumatic wounds, its terminal performance is highly dependent on entry velocity.
If the muzzle velocity of a 5.56mm rifle falls below a certain threshold, which is common in short-barreled rifles and pistols, expanding ammunition is often necessary to compensate.
● Suppressor optimization
More and more shooters choose to purchase or fabricate sound suppressors to reduce the risk of permanent hearing loss on firing ranges and in the field. The acoustic pressure level of unsuppressed gunshots is typically between 140 and 165 decibels (dB).
When firing guns recreationally and competitively, the use of proper hearing protection is essential. That being said, hearing protection on firing ranges can interfere with your ability to understand range commands and communicate with your fellow shooters. Sound suppressors, also known as silencers, allow you to shoot more safely.
With both subsonic and supersonic loads available, the .300 Blackout is optimized for use with a sound suppressor. Unlike subsonic 5.56mm ammunition, which is rare and of dubious reliability in gas-operated actions, .300 Blackout subsonic loads perform reliably. They deliver bullet weights and energies comparable to those of .45 ACP +P handgun ammunition. You may need to change the buffer to ensure reliable cycling.
● Bullet selection
As the .300 Blackout uses .308-caliber bullets, you have a wide variety of projectile options to choose from, including those with a high ballistic coefficient. This affords a greater degree of flexibility to the handloading enthusiast for load development and experimentation.
● Powder combustion
In addition to its useful sound suppressors, the .300 Blackout achieves more uniform propellant combustion in short-barreled rifles and pistols. This can reduce the extent of the muzzle flash and ensure that minimal energy is wasted.
Making the Best Caliber Choice
Both the 5.56mm NATO and .300 Blackout are extremely capable rounds that are useful for various applications. However, if you need a general-purpose or precision rifle, the 5.56mm is still the superior choice. The .300 Blackout cartridge is more beneficial for specialized applications, such as hog hunting, and when using a sound suppressor.
