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5 Best AR-15 Calibers and Cartridges for the Money [2021 Review]


The AR-15 is one of the most versatile rifles that money can buy. Its standard 5.56x45mm cartridge is in use worldwide by police and NATO forces, so you know it’s good.

Lately, it seems a popular thing in the gun world to knock the 5.56. But mostly that’s just because people want something new, even if they don’t really need something new.

So, maybe you’re looking for a bigger caliber, something with a little more bang than the 5.56 (pun intended). Never fear, there are some options for you to consider. The most common of these more powerful cartridges are:

What to Consider When Seeking an Alternative AR-15 Caliber

Making the choice to upgrade your AR has consequences and can affect lives in the real world.

It can make you look like an idiot, never mind the damage to your bank account, if you don’t choose the right caliber for the job.

But, what’s the difference between the different cartridges for the AR-15?

There are three basic categories to think about when choosing a cartridge for your AR and these three usually involve some compromises between power, cost, and usefulness.

You should also consider the flexibility and availability of rounds (the 5.56, is the most common cartridge, is readily available).


Everyone knows bigger cartridges are more powerful, right? Not necessarily.

The power of a cartridge depends upon two main factors: velocity and penetration. Though these factors are different, they’re still similar, and one affects the other.

Think of velocity like the engine of a car. It’s what gives the bullet the ability to move. Velocity is defined as: “an object is the rate of change of its position with respect to a frame of reference. Velocity is equivalent to the specification of an object’s speed and direction of motion.

Consider these velocities and weights.

image comparing the different velocities of objects

Yeah, I’m not getting math involved here, though velocity can accompany some scary mathematical equations. If you’re into math, you can research numbers like the kinetic energy of around. If you’re not, suffice it to say that velocity means motion: the bullet moving through the gap between you and a target.

Bullet weight (also called grain or gr, is a unit of measurement) is the ‘transmission’ of the car engine; it’s what gives the round the ability to move (achieve velocity). This is what causes round penetration, and for both hunting or defense, maximum penetration should be your main thought.

Why do these factors matter?

Well, light bullets at high velocities will still lack the penetration of heavier bullets moving at medium velocities. The weight gives the bullet its capability to penetrate the target.

Think of it like this: you wouldn’t drive a semi to work every day, nor would you hook up a sub-compact car to a trailer to haul it. Your power-to-weight ratio wouldn’t be sufficient to move the trailer.

So, you can see it’s not exactly easy to define the exact stopping power of any cartridge. There are numerous variables involved, but you don’t have to be a rocket scientist to figure out the basics.


This is an important factor, whether you’re on a budget or not. Of course, no single firearm can do everything, but certain guns can do most things sufficiently well. The AR-15 is one of those flexible guns.

It’s easy enough to upgrade your AR by adding new uppers and mags. Rather than wasting time (and money) getting all the possible options, you should research the available cartridges for the AR so you’re not tempted to purchase too much cartridge for the job you intend for it.

Consider that you’re probably upgrading for home defense, hunting, or competition shooting. You’re probably tempted to show off by getting the strongest round you can find. But, you’ll pay for that in extra materials, not to mention the impact of said round to your body from recoil.


It’s not a myth that when you purchase a gun, you have to feed it. This is especially true if you plan to do any target shooting; even if your AR is for home defense, you’ll want to stay sharp with it.

And that means shooting it, regularly.

You won’t want to upgrade your AR to a round that’s difficult to find. And you should take into account the cost of the round you plan to upgrade to. The more rare the cartridge, the higher the cost will be.

Consider the struggle to find ammo during a scare, when ammo is in high demand, or when there’s a low supply. Shipping ammo can be banned by states at any time, and once that happens, it can take a lot of time and effort to overturn. California gun owners are well aware of this fact. Buying ammo online has become the best way for many shooters to get their supplies.

None of the alternate AR cartridges are going to replace the 5.56. Costs of the 5.56 have come down, but they’re still the choice of NATO and police forces, so getting the rounds might always be a problem, either in cost or supply.

5 Most Common Alternate AR-15 Calibers

Let’s take a quick look at the common alternate AR rounds and see what kind of changes you’ll need to make to your AR.

Cartridge Cost of Rounds Changes Needed Uses
.300 Blackout Low Barrel Defense, hunting, target shooting
6.8 SPC Low Barrel, bolt, mags Competition, hunting
6.5 Grendel Medium Barrel, bolt, mags Competition, hunting
.458 SOCOM High Barrel and bolt Hunting
.50 Beowulf High Barrel and bolt Hunting

You should also compare ballistics for each of these AR rounds.

So, let’s take a look at these alternative AR round in greater detail and discuss the pros and cons of each.

1.   .300 AAC BLACKOUT (.300 BLK)

image of the .300 AAC BlackoutThe .300 AAC Blackout is also commonly referred to as the .300 BLK or as the 7.62x35mm. It was designed as an intermediate cartridge that could still be used in existing AR-15 type rifles chambered for 5.56. The .300 AAC was designed from the beginning to offer ballistics similar to the 7.62x39mm, like is used in the AK-47 series of rifles but while still using standard AR-15 magazines.

Of course, you may wonder why it wouldn’t be wise to just chamber an AR-15 for the 7.62x39mm to begin with. The answer is because the 7.62x39mm round comes with a cartridge taper that makes it inefficient to load in AR-15 magazines (reliability problems will result).

The .300 AAC has an effective range of around four hundred and sixty meters, with a lower velocity and bullet drop at shorter distances. However, the rounds do offer better ballistic coefficients than the 7.62x39m round does, assuming that you shoot the rounds out of similarly lenghted barrels.


The .300 Blackout offers you a wide variety of different bullet types, offers good ballistics like the 7.62x39mm (if not better), and actually permits very little velocity loss when it is fired out of a shorter barrel.

However, the factory ammunition can also be very expensive and harder to find, and as with the 7.62x39mm, it’s also less adept for long range shooting as well (as noted previously, 460 meters is considered the maximum range for the .300 AAC, but staying under the 300 meter range would be even better).

2.   6.5 GRENDEL

image of the 6.5 GrendelAs with the .300 AAC, the 6.5mm Grendel (also known as the 6.5x39mm) is another intermediate rifle cartridge designed for the AR-15. But whereas the .300 AAC was designed to be the AR-15’s answer to the 7.62x39mm, the 6.5mm Grendel was designed from the beginning to be a high accuracy and low recoil round at medium to long ranges (up to eight hundred yards).

The 6.5 Grendel has indeed proven to be a very versatile caliber, and it’s now being adopted for other rifle platforms beyond the AR-15, including AK-type rifles and various kinds of bolt action rifles.

In order to fit inside the standard 5.56 AR-15 magazines, the 6.5 Grendel was designed from the get go to be a shorter round with a larger diameter case. Factory loaded ammunition commonly weighs anywhere from ninety up to a hundred and twenty nine grains.

Ballistically, the 6.5 Grendel is superior to the 5.56x45mmm NATO, and offers better terminal energy than both that round and the 7.62x51mm NATO like is used in the AR-10 platform. A 30 round 5.56 magazine will hold 26 rounds of 6.5mm Grendel.


If you want to improve the ballistics of your AR-15, a great option will be to switch out your 5.56 upper for a 6.5 Grendel upper (the 6.5 Grendel upper will be fully compatible on a 5.56 lower). The 6.5 Grendel offers superior terminal ballistics and range than the 5.56 does, and can enhance the performance of your AR-15.

The biggest negative to the 6.5 Grendel is that it will not always feed reliably in 5.56 magazines. Most shooters report that polymer 5.56 magazines will feed the 6.5 Grendel rounds less reliably than the metal GI-style magazines will.

3. 6.8 SPC

image of the 6.8 SPCThe 6.8 SPC, also known as the 6.8mm Remington Special Purpose Cartridge, is a rimless intermediate rifle cartridge that was the result of a collaboration between the United States SOCOM, Army Marksmanship Unit, and Remington Arms as a potential replacement to the 5.56.

Obviously the 5.56 has not been replaced, but the 6.8 SPC can still be a good alternative, especially if you are using a short barreled AR-15.

The 6.8 SPC actually uses the same diameter of bullet as the .270 Winchester does. It was designed to create better ballistics for the AR-15 in the short barreled configuration, such has 10.5 inches and under, while bridging the gap that existed between the 5.56 and the .308.


The 6.8 SPC is a very devastating round that offers twice the ballistics of the 5.56x45mm NATO, but it also only offers those ballistics at shorter ranges.

The 6.8 was designed specifically to increase the terminal effectiveness of the AR-15 at limited ranges, and is therefore best suited in a CQB or short barreled rifles. In this scenario, the 6.8 undeniably excels. For shooting applications at longer distances, however, the performance of the 6.8 SPC begins to fade. The round is undergoing more testing by the United States military.

Additionally, the 6.8 SPC can be very expensive as well, so be sure to take that into account.

4.  .458 SOCOM

image of the 458 SOCOMThe .458 SOCOM round was designed due to the apparent lack of stopping power that the 5.56x45mm NATO round was offering US troops stationed overseas in Somalia. Many enemy fighters were having to be hit multiple times by American personnel armed with M4 5.56 rifles, prompting the military (and specifically members of the Special Operations command) to seek out a larger and more powerful round that could still be used in the M4 and M16 rifles in services.

The .458 SOCOM was the result, being designed in 2000 and released in 2001. The .450 SOCOM was designed to accomplish multiple roles at once: it would need to be fired at subsonic velocities when used with suppressors, it would need to be substantially more powerful than the 5.56, and it would need to be fully functional in existing AR magazines.

The .458 SOCOM is indeed fully compatible with the 5.56 AR platform, including most 5.56 magazines, the buffer spring, magazine well, and the buffer. However, the .458 round is much larger, and therefore the double stack 5.56 magazines will act as single stack magazines when they are loaded with the .458 SOCOM. A standard thirty round 5.56 magazine, for example, will hold ten .458 SOCOM rounds.


The main pro of the .458 SOCOM is its additional stopping power at the expense of range. It’s fair to compare the .458 SOCOM with the .45-70 Government round, which likewise is a mid range rifle caliber but that still packs a significant punch within its ranges.

Many Alaskan hunting guides will often opt for a lever action rifle chambered in .45-70 as defense against dangerous game such as brown bear, but many others will often go with an AR-15 chambered in .458, because it’s a semi-automatic with faster firing and reloading abilities while still offering similar ballistics.

Indeed, the .458 SOCOM is an excellent caliber to use as defense against dangerous big game. However, it also has limited magazine capacity (ten rounds will fit in a standard thirty round 5.56 magazine, as noted) and the ammunition can be pricey and sometimes hard to find.

5.  .50 BEOWULF

image of the .50 BeowulfThis is the round that started the big bore AR-15 concept. It’s said to have been developed by the same people who designed the 6.5 Grendel for military use in stopping vehicles at checkpoints as an aid to car bomb prevention. Alexander Arms produces .50 Beowulf uppers for the AR-15.

This monster round packs a punch at both ends and empties your wallet in the process. But the rounds are only produced by a few manufacturers and don’t have premium options for performance hunting.

To keep yourself in ammo, you’ll probably have to learn how to reload. If you’d like to try your hand at reloading, check out our Complete Beginner’s Guide to Reloading Ammo.


There’s no real advantage to this round for home defense; it’s just as effective as an M855 5.56 round and is a deep penetrating bullet. Likely, it would fully penetrate a bad guy and keep going.

For hunting, it’s powerful enough to take down anything in North America, but the key is to get close to your target and use bullets that open reliably to dump all its energy in your target.

As mentioned above, ammunition is rare and costly. If you really have your heart set on the .50 Beowulf, then you’d be better off learning how to reload if you want to have ammunition at all times.

Some Cartridges to Avoid

Now that we’ve discussed some excellent options for AR cartridge upgrades, let’s talk a bit about cartridges to avoid, because there are some of those, too.

6. 45X39MM

This was Russia’s answer to the 5.56 M16 in the battle for the AK-74. It’s been maiming soldiers since the 1970s and has a horrible reputation.

When used by some bad guys in Afghanistan, these rounds blew up or ricocheted when they hit something like a wall. They didn’t penetrate well, nor did they kill efficiently; they were more likely to cause superficial wounds.

Years ago, the market was flooded with this ammunition, but that source has now dried up. Changing your AR to this configuration won’t gain you any benefits, except possibly cheaper ammo. But overall, it gives you a less effective firearm.

7. 62X39MM

This true .30 caliber cartridge fires its 120gr bullets at close to 2200fps from a rifle barrel. While this sounds ideal, it’s really not.

It’s a tapered casing and requires an exaggerated, curved magazine and custom lowers to perform well. It also uses steel-cased ammo with a polymer coating; this doesn’t work well in direct impingement weapons that use the gas from a fired round to cycle the action and reload the next round.

This round isn’t any better than the .330 Blackout and you can’t find it in as many loads. It’s also not easily suppressed, so you’re probably better off going to the .300 Blackout.

But if you’re completely sold on the 7.62, make sure to find a custom lower receiver that moves the pistol grip back to accommodate the magazine required.

Final Thoughts

When you’re looking to upgrade your AR-15, you’ll need to consider a compromise between power, cost, and availability of parts and ammunition.

Some AR upgrades come with a high price tag, so you might need to learn about reloading ammo if your heart is set on these cartridges.

In addition, some of these newer cartridges may fade away in the future, so you may end up doing another upgrade if your rounds disappear.

Decide what’s the most important job you’re upgrading for: is it hunting, competition, or home defense?

The answer to this question will get you closer to choosing the right cartridge upgrade for your AR-15.



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