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The Best AR-15 Cartridges (And Why)

The Best AR-15 Cartridges (And Why)

Posted by on Oct 25th 2021

The AR-15 can chamber a lot of cartridges. That includes pistol calibers. In short, it can be a real pain deciding which cartridge you'll put in your first, or next, AR-15 build. We're going to help you decide by covering the most popular cartridges by specific use, which includes:

  • Deer hunting
  • Home defense
  • Short barrel builds
  • Suppressors
  • Varmint
  • All purposes

Criteria for judging calibers

Picking the perfect caliber for hunting, defense, or some other purpose requires judging that caliber based on its overall power, velocity, max effective range, recoil, cost, and the setup required of the AR-15 itself.

Now, let's cover the most popular cartridges.

Best AR caliber for self defense

The AR-15's become one of the most popular home-defense firearms in existence. It's easy to shoot and control, it's light and compact, and it provides high capacity for virtually every caliber that it can chamber.

Before we get into which caliber's best for home defense, it's important to talk about over-penetration: The risk that a stray round can cause collateral damage, even injury or death to a loved one, in the event you have to fire at an intruder in your home. The original Box O' Truth spells it out quite clearly, with evidence and testing: Any caliber capable of eliminating a threat in the home is also capable of over-penetrating that home's walls.

This risk can only be reduced, but not eliminated. With that in mind, what's the best AR caliber for home defense?

9mm (The AR-9 pistol)

It's little wonder why the 9mm cartridge is the most popular handgun caliber among law enforcement and military units worldwide. It's powerful, accurate, and optimized for self defense. The 9mm's recoil and muzzle energy are easy to manage even for novice shooters. Its risk of over-penetration is much lower than comparable handgun cartridges, and it boasts high velocities that allow it to remain accurate at distances beyond 100 meters.

It makes sense that chambering 9mm in the AR platform - called the AR9 - makes one of the best personal defense weapons imaginable. An AR9 can easily provide up to 40 rounds of ammo in a single magazine while remaining light and maneuverable. It's way more stable than any pistol, and its accuracy is greater than any what handgun could provide.

Optimal Setup

The 9mm cartridge burns its powder rapidly. It's designed to be fired from barrels as short as 3.5" to 4", so your AR9's barrel doesn't need to be rifle-length. Building an AR9 pistol makes sense, since you can maximize the velocity of most 9mm loads with a barrel as short as 9".

Read the AR9 Pistol Guide here.

The best AR caliber for suppressors

Shooting suppressed isn't all about being tactical, or hunting hogs at night. It can be enjoyable simply because it eliminates the need for hearing protection -- as long as you're shooting the appropriate ammunition. And no cartridge makes better use of a suppressor in the AR-15 than a certain .30 cal.

300 Blackout

Often called 300 BLK, the 300 Blackout cartridge was developed for the AR-15 platform. It was made for special operations forces, who wanted a new M4 round that could compete with the likes of the 7.62x39mm. Spec ops also needed this new round to provide subsonic ballistics, so that it could be effectively suppressed for night ops.

Unsurprisingly, a round capable of doing all these things found popularity in the civilian AR world, and now 300 Blackout is one of the most popular AR-15 calibers. In fact, this writer built an AR-15 short-barreled rifle in 300 BLK just so he could test out his homemade suppressor. The rifle in question is this ACOG'd beauty:

With this rifle's setup, this writer is able to shoot subsonic 300 BLK loads at around 122 decibels. That's quiet enough to comfortably shoot without ear pro all day long. In fact, the cycle of the bolt carrier group is often perceived to be louder than the shot report itself.

Optimal Setup

That kind of sound suppression makes for impressive performance for any AR caliber. But to reliably cycle subsonics and supersonic loads, you need to make sure you have a barrel that's long enough, a buffer that's light enough, and a gas system that's short enough.

What does that look like? An optimal 300 Blackout AR sports a 9" to 11" barrel (the rifle above uses a 10.3" barrel), with a pistol-length gas system and ideally an adjustable gas block. Lastly, a standard Carbine (3.0 oz) buffer should work perfectly. Engineers designed the 300 BLK to make use of the 5.56/.223 buffer setup on purpose, to help the military save money on parts.

Read the 300 Blackout Build Guide here.

For General Purpose and Varmint

The AR-15 is an all-around great rifle. It's capable of shooting accurately up close and far away. It's great for hunting, personal defense, competition, and general sport. Not surprisingly, the O.G. calibers -- 5.56 NATO and .223 Remington -- are the ones to shoot if you want an effective, general-purpose AR-15.

Side note: These two cartridges, for the sake of performance, are identical. They provide the same ballistics and ARs chambered in either cartridge use all the same parts. One's designed for military rifles, and one's made for the civilian market. Both, however, are sold to civilians for roughly the same price.

5.56 NATO/.223 Remington

So, why are the old and dated 5.56 NATO and .223 Remington loads still the most popular cartridges found in the AR-15? For starters, they're small and incredibly fast rounds. That means they provide high amounts of energy and accuracy downrange, in a small package that provides relatively low volume and light recoil. In fact, few other rifle cartridges can provide the kind of lethal accuracy that 5.56/.223 does, using a bullet that's only a bit larger than a .22 LR.

And up close, these rounds are incredibly deadly. Either load will penetrate deep, often tumbling and creating a large wound channel. And, surprisingly, these rounds don't over-penetrate hard structures and walls as much as other rifle and handgun cartridges. Upon striking wood, brick, concrete, and even drywall, 5.56 NATO and .223 Remington have a greater chance of fragmenting or tumbling, quickly dissipating their lethal energy.

All these characteristics make the 5.56 and .223 the best all-purpose loads for the AR-15, to this day. They're also arguably the most popular varmint loads ever developed. After all, .223 Remington was originally marketed to American sportsmen for varmint hunting decades ago.

Optimal Setup

The optimal setup for a 5.56/.223 gun is convenient because the optimal barrel length happens to be the minimum legal barrel length: Sixteen inches. At this length, you're getting optimal velocity in the shortest possible rifle configuration.

Here's the ultimate guide on how to build an AR-15.

For Long-Range Shooting

Long dead is the idea that you need to buy or build the AR's larger .30-caliber platform to enjoy some good, accurate long-range shooting fun. The AR-15's original .22-caliber platform has been souped up to provide accuracy at up to three times its usual effective range. This is accomplished using a particularly hot 6.5mm cartridge called the Grendel.

6.5 Grendel

The 6.5 Grendel was made by Alexander Arms to replace 5.56 and .223 for long-distance shooting. The cartridge works on a simple principle: Fire a slightly larger and heavier bullet with a lot more gunpowder. The result is a load that provides a remarkably high ballistic coefficient. That's the ability to overcome air and wind resistance in flight. The Grendel load is so accurate that it allows the AR-15 to reliably strike targets at 800 yards or more.

Optimal Setup

The average Grendel travels about as fast as the average 5.56 load. But these rounds are heavier and to obtain that velocity with a heavier cartridge, you need a longer barrel. 6.5 Grendel tends to reach optimal velocities using a barrel that measures 20" to 22".

Read the 6.5 Grendel Build Guide to learn more.

For Hunting American Game

Accuracy is argued to be the most important factor contributing to a hunter's success. But energy is just as important for stopping that game from making a run while wounded. So, how does the AR platform optimize itself and provide accuracy with power? Simply take the 6.5 cartridge and add more powder.

6.5 Creedmoor

That's exactly what Hornady did in 2007 by creating the 6.5 Creedmoor. The Creedmoor fires the same, exact bullet as Grendel but enjoys a significant increase in velocity. That's because this cartridge's shell casing is sized up for the AR's larger .30-caliber variant. We're referring to the AR-10 and its modern,commercial equivalent, the DPMS LR-308.

Optimal Setup

Like Grendel, the Creedmoor shell casing burns a large amount of powder. All that expanding gas needs space to accelerate the round, so a long barrel is required: A 20" to 22" barrel gets the job done, but velocity is truly maximized with a 24" barrel. Besides the bolt and barrel, the Creedmoor-chambered AR uses all LR-308 components.

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