What is the Caliber of AR 15 Rifles?

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Unlike a specific gun, AR 15 is a platform. This implies that one can use various parts and make it shoot several calibers. Though the original AR 15 caliber is 5.56 or .223, the platform is currently chambered in several other calibers. In fact, some calibers are designed explicitly for AR 15 rifles.

In simple terms, AR 15 caliber can literally be anything you think it can be. The most popular chambering include.300 Blackout, 9mm and.22LR. With a fat wallet, some custom AR builders will deliver a gun in any configuration of your preference.

The Originals: The 5.56 and .223

As mentioned earlier, any AR 15 caliber arose from the immemorial 5.56 and the .223. The 5.56 is light and bullets are in the typical range of XX grains. Additionally, the rounds leave the barrel at high velocities of approximately 3,000fps (feet per second).

A typical AR 15 mag carries a whopping 30 rounds, and they are reasonably light. One can actually take with him a few hundreds of 5.56 rounds with a lot of ease. This makes it favorable for soldiers.

On the other hand, the .223 rounds are meant for civilian sales and thus regulated by the SAAMI. 

Although it is mostly used interchangeably with 5.56, the features of the two are not similar, as most people argue. Despite their closeness, 5.56 is somewhat a more powered .223. However, their outsides are just but identical.

In this case, one can use a 5.56 round in a .223 chambered rifle, but the reverse is not advisable. There is a fear that .223 might not effectively handle increased pressure from the 5.56 since it’s slightly more powerful. The additional stress can cause some problems, although not a critical failure.

Most people prefer the .223 since there are various rounds readily available, with some at decently low prices.

Most Common AR 15 Calibers Alternatives

.300 AAC Blackout

It is also popularly referred to as 300 BLK or as 7.62×35mm.  Designed as an intermediate cartridge, .300 AAC Blackout is still compatible with the existing 5.56 chambered AR 15 type rifles.

 From the beginning, .300 AAC Blackout was developed to offer similar ballistics to that of the 7.62x39mm. For instance, it is used in the AK-47 rifle series while yet using standard AR-15 magazines.

The .300 AAC has an effective range of approximately 460 meters. It is also characterized by a relatively lower velocity and shorter bullet drop. However, when compared to the 7.62x39m, .300AAC rounds provide better ballistic coefficients. When contrasting the coefficients for both, there is an assumption that round shots are from barrels of similar lengths.

More than often, people wonder why it is not wise to chamber an AR-15 for the 7.62x39mm. This is because 7.62x39mm rounds come with cartridge tapers, which makes them inefficient for AR 15 magazines. Otherwise, it would result in reliability problems.

.300 Blackout Pros and Cons

This caliber offers a wide variety of various bullet types. It is also ideal since it provides ballistics that are even better than that of the 7.62x39mm. Besides, it allows insignificant velocity loss when fired from shorter barrels.

On the contrary, factory ammunition is costly and hard to find. Also, similar to the 7.62x39mm, .300 Blackout is less proficient for long-range shooting. The 460 meters mentioned earlier are on the higher end, and hence sticking to less than 300 meters is better.

6.5 Grendel

It is also commonly referred to as 6.5x39mm. Just like .300 AAC, 6.5mm Grendel is an intermediate rifle cartridge configured for the AR 15. While the .300 Blackout was developed to complement the 7.62x39mm, 6.5mm Grendel was primarily designed as a low recoil and high accuracy ammunition for medium to long ranges – up to 800 yards.

Since its development, the 6.5 Grendel has been a very versatile caliber.  As a result, it is currently being embraced for other rifle platforms after the AR 15. This includes different types of bolt action rifles and the AK-type rifles.

In the ballistic regard, the 6.5 Grendel is significantly better than the 5.56x45mm NATO.

The 6.5 Grendel Pros and Cons

The 6.5 Grendel upper is entirely and perfectly compatible with a 5.56 lower. Therefore, if planning to improve your AR 15’s ballistics, replace the 5.56 lower with the 6.5 Grendel upper. It also enhances performance since its range and terminal ballistics are superior to those of a 5.56 lower.

On the downside, the biggest challenge with the 6.5 Grendel upper is that it doesn’t feed reliably in 5.56 magazines. Most reports indicate that polymer 5.56 magazines feed on 6.5 Grendel bullets less reliably than metal GI-style magazines.

.458 SOCOM

Its development was primarily triggered by the lack of stopping of the 5.56x45mm NATO experienced by US troops in Somalia. As witnessed, many enemies had to be hit several times by the American soldiers armed with M4 5.56 rifles. This prompted the search for a larger and more powerful round that would still get loaded in M16 and M4 rifles.

As a result, the .458 SOCOM was designed and released in the years 2000 and 2001i, respectively. It was to accomplish multiple roles simultaneously. Hence, it was to be significantly more powerful than the 5.56 and manage subsonic speeds when fired suppressors. It also needed to be fully compatible with the already existing AR magazines.

Today, the .458 SOCOM and the 5.56 AR platforms are entirely compatible. This includes most 5.56 magazines, the buffer, buffer spring, and magazine well. In contrast, .458 bullets are much bigger. As a result, a double stack 5.56 magazine is used as a single stack magazine when loaded with .458 SOCOM. For instance, a standard 30-round 5.56 magazine holds 10 .458 SOCOM rounds.

Pros and Cons of .458 SOCOM

The main advantage that comes along with the .458 SOCOM is the additional stopping power though at relatively lower ranges. It compares with the .45-70 Government ammunition that packs a significant punch but in mid ranges.

When hunting or self-defense against big game, several Alaskan hunting guides advocate for .45-7 chambered lever action rifles. However, many others will opt an AR-15 chambered in .458. This is particularly because it is semi-automatic and has faster reloading and firing abilities and still offer similar ballistics. 

Although the .458 SOCOM is an excellent caliber, it has a limited magazine capacity due to its size. To be precise, only 10 rounds can fit in a standard 30-round 5.56 magazine. This ammunition can also be hard to find and fairly expensive. 


This monster round began the big-bore AR 15 concept. .50 Beowulf is believed to have been developed by 6.5 Grendel designers. Its main objective was to stop vehicles at checkpoints by the military to help in car bomb prevention.

The notorious ammunition packs punch at both ends. It is also not wallet sensitive. These rounds are only manufactured by a few firms and thus no premium options for performance hunting. To enjoy this ammo, one should learn how to reload.

50 Beowulf Pros and Cons

Though there is no much advantage attributed to 50 Beowulf, it is a deep penetrating round. It can effortlessly penetrate through a bad guy and continue going. Its effectiveness compares to that of the M855 5.56 bullet.

50 Beowulf is ideal for hunting. In fact, it can take down literally any game in North America. The only secret is to maintain a close range to your target. Also, go for rounds that can open reliably to dump all the energy into the target.

The cons of the 50 Beowulf are that it is a rare and pocket draining ammunition. Therefore, if you really have a heart for it and want to have the rounds all the time, learn how to reload.

The 6.8 SPC

6.8 SPC is a rimless rifle cartridge developed as a 5.56 potential replacement. It is also known as the 6.8mm Remington Special Purpose Cartridge.

Even though the 5.56 is still not yet replaced, 6.8 SPC is a perfect alternative mainly when used in a short-barreled AR 15.

In diameter size, it uses rounds similar to that of the .270 Winchester. It is designed to offer better ballistics for the short-barreled configured AR 15, especially those below 10.5 inches. This was an attempt to bridge the gap between the .308 and the 5.56.

Pros and Cons of the 6.8 SPC

6.8 SPC is a highly destructive round with double the ballistics of the 5.56x45mm NATO.

It was specifically designed to enhance the AR 15’s terminal effectiveness at limited ranges. It is, therefore, perfect for COB and rifled with short barrels.

Although the 6.8 SPC is a devastating round, its major drawback is that it only excels in shorter ranges. Otherwise, its performance fades.

Most Popular Rifle Accessories

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When looking forward to upgrading your AR 15, there are a variety of calibers to choose from. All you need to consider is a compromise between cost, power, and the availability of parts and ammunition. For rounds that are a bit costly, the hack is to learn reloading the ammunition.

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