Best AR Cartridges and Calibers – What to Use and When

One of the biggest draws of the AR platform is its ability to be customized to the end user’s preferences.  The ability to use a variety of calibers in one platform is part of the appeal, as each has different strengths and weaknesses, allowing the user to pick and choose what is best for them based on their individual needs.

With over 100 different calibers available to choose from between the AR-15 and AR-10 platforms, diving into each of them in detail would be longer than one article; instead, I’m going to cover some of the more popular calibers, their strengths, and ideal applications, any drawbacks they might have, and what the best AR cartridges and calibers might be for your specific situation.

As always, use this article as a jumping-off point to research anything that interests you, as forming your own conclusions is something each user should be doing.

Best AR Cartridges and Calibers – Reloading and Availability

Before jumping into the calibers themselves, I wanted to address the topics of reloading and the availability of ammo.  Before COVID-19, ammo prices and availability, along with reloading components, were in a much different state than they are now.  While there has been a bit of a surge in ammo availability lately, many reloading components are still difficult to source, so when reading this article, bear in mind that any discussion of reloading a round or availability will be in terms of “the gold old days” when things weren’t scarce.

Best AR Cartridges and Calibers – Reasons to Rimfire

.22LR has been a longtime favorite for the AR platform due to the relatively low cost of ammo as well as low recoil while shooting. This round is a great plinking and target shooting round and is great for teaching newer shooters looking to get into shooting sports.  This round is also great for use hunting small pests and varmints. The cost of the ammo is relatively low, even by today’s standards, and is widely sold. 

Though it isn’t as common as other calibers, you can reload .22LR with specific kits, though it requires some substantial work. .22LR is not recommended for defensive use though, so keep that in mind.

Best AR Cartridges and Calibers – Pistol Calibers

The AR platform has seen a ton of popularity with pistol calibers, most notably 9mm, though .45 ACP, .50 AE, and several others have seen use. Pistol calibers will have all the strengths and weaknesses they would in a pistol, with a few points to add on. 

Any pistol caliber round will require specific guns to use (instead of swapping just a part or two like with .22LR), though they can be relatively easy to tune. Pistol calibers are great for target shooting, small pests and varmints, training, and with proper ammo selection, defensive use.

Pistol calibers will vary in availability and cost depending on the specific ammo selected. Reloading of pistol calibers is very popular and pretty straightforward, with a wide range of components offered on the market to support it. Just keep in mind that different types of ammo will have different applications and plan accordingly, or you may face some unintended consequences (over-penetration of ball ammo, using more expensive hollow points on paper, etc.).

Best AR Cartridges and Calibers – The Gold Standard: .223/5.56

.223 was the original round for the AR-15 platform, with 5.56 being the original M16A1 cartridge.  The two rounds are nearly identical, with gas pressures being one of the biggest differences. These two rounds are the jack of all trades in terms of AR calibers, being used for everything from target practice and hunting to self-defense. 

There are a very wide variety of rounds available in .223/5.56 ranging from ball ammo to hollow points, barrier blind, and ammo with actual penetrators (looking at you 855A1). The strengths and weaknesses here are going to be relative to the ammo selected as well as the configuration of the gun it is shot from. 

A great example of this point is the MK12 rifle utilizing an 18” barrel with a 1:7 twist rate excelling with 77-grain ammunition like the Black Hills mk262 while underperforming with 62-grain M193 or M855/M856 (a failure of the ammo, not the rifle).

A wide variety of ammo being produced means a wide variety of availability in the market.  Not all .223 or 5.56 are created equally, with some having poor ballistics and others being produced haphazardly. 

My strong recommendation is to try a variety of brands and grain weights with your gun to find out what works best and do a ton of research on the topic.  Reloading components will follow the same mindset.  There are a ton of projectiles available in all manner of weights, shapes, and tips with almost as many powders to follow.  Use reliable load data, select your components carefully, and be smart.

Best AR Cartridges and Calibers – 300 Blackout

Not as popular as .223/5.56, due in part to the cost of the rounds (and for a while, lack of decent parts from manufacturers), 300 Blackout is something that shouldn’t be overlooked. Packing a .30 caliber round into a necked-down 5.56 casing offers some impressive results at shorter distances and there is a large selection of reloading components available. That isn’t to say that this round won’t hit targets at range, but it isn’t the main purpose of 300 Blackout, nor where it performs best.

300 Blackout is a great round for hunting small and medium game (deer and hogs mostly), useful for home defense, and can be used for target shooting, though it is expensive. More than most others, ammo selection is going to play a huge part in your success.  300 Blackout is available in both supersonic and subsonic, with rounds breaking down further into hollow points, barrier blind projectiles, game loads, tracers, and more. 

What round you select will have a lot to do with the barrel length you are running, what your application is, whether or not you are suppressing your gun and other factors.  I can tell you that if you plan to run 300 Blackout for personal protection, subsonic rounds fired from a suppressor will yield more over-penetration than supersonic rounds will, so keep that in mind before you sail a .30 caliber projectile through your neighbor’s drywall at subsonic speeds.

Best AR Cartridges and Calibers – 7.62×39

I’m going to preface this section by saying that 7.62×39 in and of itself is a solid round with hunting applications, target shooting, and personal protection (though over-penetration is a big concern)…..just not in the AR platform. Can it work? Yes, though it faces some big limitations and the list of companies making quality parts for x39 ARs is dwindling.

If you insist on going this route, save yourself some hassle and just get a CMMG Mk47 Mutant (which has been rebranded to the Resolute line if I’m not mistaken).

There are three main issues that the AR platform faces when trying to run 7.62×39. First, the case angle of the round causes some feed issues in the platform, especially when receivers are altered to try and take AK magazines. To address this, a new AR magazine was developed, but many on the market still have issues with reliability in terms of feeding and are seen as low-quality items.

Lastly, the larger diameter of the case head requires the bolt face to be thinned out, leading to structural integrity issues and excessive wear on the extractor (not to mention the harder primers requiring an enhanced firing pin to reliably cause rounds to detonate).

The round itself has great application in several areas. Hunters have been using 7.62×39 since the 90s on game up to the size of whitetail deer (it is suited for anything that you could use .30-30 Winchester to hunt). Relatively inexpensive steel-cased rounds are good for target shooting as well. 

Personal protection is an option, though due to the ballistic performance of the round, as well the fact that many versions of the round use a bi-metal projectile, lead to concerns of over-penetration.

Best AR Cartridges and Calibers – 6.5 Grendel

Designed as a low-recoil, intermediate cartridge that is highly accurate at mid to long-range, 6.5 Grendel has seen an upsurge of popularity in the past few years as a great hunting round. Since it was released from trademark in 2010 and standardized under the SAAMI specs, a wider variety of ammo selection has hit the market, with offerings from many of the big-name manufacturers. Since that time, a variety of reloading supplies has also hit the market, allowing shooters to create some truly impressive hand-loads for accuracy.

6.5 Grendel is a cartridge worth considering for precision shooting, medium to long-range target shooting, and hunting up to medium-sized game (think great for hogs and large deer, not ethically great for use on elk or bears).

With rounds staying supersonic as far as 1,200 yards in testing, the Grendel rounds offer an appealing performance from a standard AR-15 package with only minimal part swapping (bolt, barrel, and magazine namely). The round is ill-suited for self-defense as it is designed to be a long-distance round.

Best AR Cartridges and Calibers – 6mm ARC

The newest round on the list, 6mm ARC was introduced in 2020 and has caused quite a few heads to turn since.  6mm ARC was designed as a low-recoil, long-range cartridge focused on high accuracy.  It requires a dedicated bolt and barrel, though 6.5 Grendel magazines can be used.  

This round is still very new, so applications have been limited to medium to long-range target shooting and some hunting (mostly deer).  The biggest limitations right now are the lack of industry support in terms of rifle components, ammo selection, and reloading components, though more and more are being added all the time.

Best AR Cartridges and Calibers – Big Bore Calibers

The Ar-15 has several big bore calibers that have gained popularity, the three most popular at the time are .450 Bushmaster, .458 SOCOM, and .50 Beowulf.  Big bore rounds are great for hunting, have some target shooting application, but are mostly ill-suited for personal protection. .50 Beowulf has seen an increase in use for hunting deer, moose, and black bears inside of 200 yards.

.450 Bushmaster was designed to kill big-game at a range of 250 yards without needing multiple shots and is popular for hogs, some bears, deer, and elk. It is also worth noting that as a straight wall cartridge, it is legal in states with a ban on bottleneck cartridges, which is part of its popularity. .458 SOCOM is useful against the same targets .450 Bushmaster is but will have a bit more rise on shots than the flat-shooting Bushmaster.

Best AR Cartridges and Calibers – .308/7.62x51mm NATO

Let’s get this out of the way now, according to SAAMI, .308 and 7.62x51mm are interchangeable. .308 is arguably the most popular hunting cartridge in the world, as it sees use on most continents across the globe for hunting medium to large game. The Alaskan Department of Game and Fish even recommends this round for hunting brown and grizzly bears, something big bore calibers in the AR-15 cannot boast.

With the insane popularity of the round and the widespread availability of the AR-10 platform, there are a great many types of ammo to choose from for .308.  Everything from specialized hunting rounds to soft-tip hollow points are commercially available, with even more components purchasable for reloading. If 5.56 is the cartridge by which all other AR-15 rounds are measured, .308 is the same for the AR-10.

Best AR Cartridges and Calibers – 6.5mm Creedmoor

6.5 Creedmoor focuses on distance and speed over raw energy when compared to .308. Supersonic out to over 1,100 yards with a 140gr projectile, 6.5 Creedmoor has become a precision and long-range shooting favorite in recent years. Reloading components have been widely available in a large selection, while high-end factory ammo has also seen incredible results in terms of accuracy and performance. This round is great at distance, but it does lose a bit of kinetic energy when compared to .308 so it is suited for small to medium-sized game (like deer, hogs, or coyote but not elk or moose).

Best AR Cartridges and Calibers – Conclusion

Despite touching on several calibers across the AR-15 and AR-10 platforms, we haven’t even scratched the surface of what is available. To find the round that best meets your needs, you need to figure out what those needs are.

  • Will be you using the round for personal protection?
  • What distances will you be shooting?
  • Are you buying factory ammo, or did you want to hand load your rounds?
  • What is available in the market where you are? Remember, finding ammo isn’t as easy as it was a few short years ago.

Once you have an idea of what you need the round to do, figure out the best setup to take advantage of the strengths the cartridge offers, while minimizing its downsides, and you have a game plan for success. From there on out, training and proper repetition are going to be the deciding factors on how well the calibers you choose perform for you.

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