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From a logistics, training, and effectiveness standpoint, the 5.56 NATO (5.56x45mm) is an excellent choice for your AR-15 to be chambered in. It is also the best bang for your buck affordability-wise. However, the 5.56 NATO has been around since 1957 and while it has evolved throughout the decades, newer cartridges have since been introduced that exceed its performance while still working in the AR-15 platform. Here are some cartridges that may be worth considering if the 5.56 NATO is not providing you with the results you desire.
The most common alternative that I enjoy is the 300 AAC Blackout (also known as 300 Blackout or 300BLK). This cartridge began production in 2010 and was adopted from the 300 Whisper cartridge with some slight variations. The biggest difference between the 5.56 NATO and the 300 AAC Blackout is the use of a 30 caliber projectile. In fact, it can even use the same bullets as a 308 Winchester depending on the loading. It provides a wider range of bullet designs and performance where you can find grain weights from 110 to 220 grains from quality manufacturers. Performance-wise, the biggest deviation of the 5.56 NATO and the 300 AAC Blackout is the ability to use the cartridge suppressed while still maintaining good terminal ballistics. Subsonic cartridges with heavier bullet weights are extremely quiet and thanks to the projectile design, are still effective on the target. Meanwhile, the 5.56 NATO is a velocity-dependent cartridge that will not perform properly at subsonic speeds of less than 1,126 feet per second. The 300 AAC Blackout is incredibly quiet when used in combination with a suppressor and subsonic loadings, even outperforming the 9mm Luger (another commonly used cartridge for suppression). I like to shoot Winchester 300 AAC Blackout 200-Grain JHP rounds as they’re affordable and let me train at night without bothering my neighbors.
300 AAC Blackout also offers a wide array of supersonic loadings that can be very effective for hunting or self-defense. Its ballistic performance has often been compared with the 30-30 Winchester or 7.62x39, both of which are well regarded. Although one of the best attributes of the 300 AAC Blackout is that it performs well out of shorter barrel lengths. You can get close to full powder burn out of a 9” barrel, so your supersonic cartridges will still perform out of a pistol or a short-barreled rifle setup. My supersonic cartridge of choice for self-defense and hunting is the Federal Fusion MSR 300 AAC Blackout 150-Grain, as my 9” barrel has a 1:5 twist rate and prefers heavier projectiles. Due to the heavy bullets and limited capacity for powder, the supersonic 300 AAC Blackout loadings are considered to have a maximum effective range of 460 meters out of a 16” barrel, according to the US Armed Forces. However, the limited capacity of powder has a major benefit as well.
The 300 AAC Blackout’s parent cartridge is the 5.56 NATO and even uses the same 223 Remington case that has been necked down to accept the 30 caliber projectile. This means that it can use the same magazines and bolts that your 5.56 NATO AR-15 uses, which is one of the biggest reasons why the 300 AAC Blackout is the most common alternative. With magazine and bolt uniformity, exchanging between the two chamberings only requires a barrel swap so the 300 AAC Blackout is a great place to start if you want to try something else. Plus, if you have a suppressor or just want a shorter overall length for your firearm, it’s tough to beat this kind of ammo.
6mm ARC (6mm Advanced Rifle Cartridge)
The 6mm ARC (6mm Advanced Rifle Cartridge) was first introduced in 2020 and developed by Hornady in conjunction with the U.S. Department of Defense. It was originally created to push the boundaries of what could be accomplished within the AR-15 platform, and the 6mm ARC’s greatest advantage is its long-range capabilities. Utilizing a 6mm projectile provides the cartridge with an excellent ballistic coefficient (a bullet's ability to overcome air resistance) when compared to the 22 caliber 5.56 NATO projectiles. For example, the 6mm ARC was designed around the 108-grain bullet weight for optimal performance. The Hornady 6mm ARC 108-Grain ELD Match uses a bullet that has a 0.536 ballistic coefficient, whereas the IMI 5.56 NATO 77-Grain Sierra MatchKing uses a bullet that has a 0.373 ballistic coefficient. That’s a drastic increase in long-range capability in bullet design alone—which helps to maintain a flat trajectory during flight.
Another advantage of the 6mm ARC is the amount of energy it puts out. If we compare the two previously mentioned cartridges, the Hornady Match 6mm ARC has a muzzle energy of 1813 ft/lbs. while the IMI Razor Core 5.56 NATO has a muzzle energy of 1283 ft/lbs. It's a significant difference in energy, which is quite impressive considering the difference in recoil does not directly correlate. The 6mm ARC is said to have 30-35% less recoil than a 308 Winchester but performs better at 1000 yards due to velocity and air resistance. Although it does recoil more than the 5.56 NATO, you get a lot of performance in return. This additional energy will be useful at long range just as much as it will close range for defensive purposes.
The 6mm ARC cartridge does require additional parts in order to work properly in the AR-15. Its parent case is the 6.5 Grendel, which means you will need a different bolt as it has a wider base. This also means a different magazine will be needed, which has slightly less capacity than a 5.56 STANAG magazine. Although the difference is minimal considering a 5.56 STANAG magazine has a 30-round capacity and a 6mm ARC magazine of similar size has a 26-round capacity, magazines and bolts that work for 6.5 Grendel will also work for the 6mm ARC so they are readily available.
Introduced in 2019, the 350 Legend is designed as a deer, hog, and black bear hunting cartridge that meets the specific hunting requirements set by states such as Michigan, Ohio, Iowa, and Indiana. These regulations allowed the use of 35 caliber or larger projectiles with straight-walled cartridges that have a minimum case length of 1.16 inches and a maximum length of 1.8 inches. The idea behind these design parameters is to give hunters cartridges that have a limited range to avoid unintentional property damage or injury to nearby residents. To accomplish this, the 350 Legend uses the 223 Remington as its parent case and forms them to accept .357” diameter bullets. As a result, it has earned the title of the fastest SAAMI-approved straight-walled hunting cartridge, producing a muzzle velocity of 2,325 feet per second out of a 20” barrel while packing 1,800 ft/lbs. of energy. Even with this increase in velocity and energy on target, it's important to keep in mind that it is a shorter-range cartridge and the 5.56 NATO will outperform it in long-range applications.
The best benefit of using the 223 Remington as the parent case for the 350 Legend is its ability to be adapted to the AR-15 platform. Not only does this provide hunters with an accurate, lightweight, and easy-to-use firearm, but it also gives anyone using an AR-15 for self-defense an interesting ammunition option. Thanks to the .357” bullet diameter, there are a ton of different loadings that offer good hunting performance, threat elimination, and even subsonic capabilities; all while producing a lot more energy than the 5.56 NATO with comparable recoil and muzzle blast. Unlike the 300 AAC Blackout (which can also use the 223 Remington as its parent case), the 350 Legend requires its own proprietary magazines due to the difference in cartridge size. It seems that most of its magazines max out at 20 rounds like the DuraMag SS Magazine AR-15 350 Legend as they maintain a straight construction throughout the length of the magazine. Although the ammunition selection and increased energy are impressive for such a relatively new cartridge, there is another benefit that may be more important.
I do not personally own 350 Legend, but the reason it has been on my radar has nothing to do with hunting restrictions or the increase in energy of the cartridge—it has everything to do with logistics. During the ammunition shortage of 2020 and the height of the pandemic, the 5.56 NATO was flying off the shelves. It became downright impossible to find 5.56 NATO unless you were willing to pay exorbitant prices. However, one AR-15 ammunition type I could always find was the 350 Legend. At the time, I assumed it was due to lack of popularity (which did play a role) but it was more than that. The cartridge is built around two of the most popular ammunition types in the USA: the 223 Remington and 9mm Luger. Although it can load .355” bullets (the same as a 9mm Luger), it requires a different bullet construction. Regardless, this means that most of the machines and tooling setup for 223 Remington and 9mm Luger can be easily converted over to 350 Legend. You can also see this logistical win based on the prices of ammunition. You can get 20 rounds of Winchester 350 Legend 145-Grain FMJ for under $15.00, which is barely more expensive than the 5.56 NATO M855. Due to its prevalence during the last ammunition shortage and its current affordability, I would say that the 350 Legend is worth being on your radar for AR-15 cartridges, especially if longer ranges aren’t a concern.
Conclusion
There are many other AR-15 compatible cartridges out there that deserve an honorable mention such as the 6.5 Grendel, 6.8 SPC, 450 Bushmaster, and 458 SOCOM. However, if I were to build an AR-15 and I didn’t want to go with 5.56 NATO (which is what I would recommend in most circumstances), I would only consider the 300 AAC Blackout, 6mm ARC, or the 350 Legend as an alternative. They provide you with everything an AR-15 is capable of while still having the logistics needed to support them. However, it will only be a matter of time before a new cartridge appears and we all decide it's the new best round ever, and then we’ll go right back to the 5.56 NATO.
-- Magic Prepper
