What is the best caliber for the AR platform?

The AR in AR-15 or AR-10 stands for the company who designed the rifle, ArmaLite. People typically assume it means assault rifle. They come in many different calibers. But which is the best caliber for you?

S&W M&P 15 AR-15

What many do not know though is that there are multiple versions of the ArmaLite rifle. There is the AR-15, AR-10, and larger versions of the platforms to accomadate specialty ammunition. In today’s guide, we’ll go through a small range of your caliber possibilities based on your uses and the ammunition’s availability.

You can see that the gun’s modularity allows end users to interchange pieces and parts without help from a gunsmith.

What calibers can you build an AR-15?

You might ask this question assuming it’s just a 5.56 caliber rifle. If you’re a little more educated in the rifle and gun industry, you’d know that it’s a 223 Remington or 5.56 cartridge that can go in it. Pulling the layers back a bit further, you’d be surprised to find you can actually use many different calibers in the platform.

Here’s our review for AR-15 lowers.

The AR-15 is best known for its 223 Remington and 5.56 calibers but it can also house a .300 Blackout round. Okay – well that’s sort of common knowledge.

5.56 AR-15 rifle cartridge

But did you know that you can switch out the upper receiver with a larger one to shoot .458 SOCOM, .450 Bushmaster and .50 Beowulf? What? Did I lose you there? That’s correct – the AR-15 is not just for 223/5.56 and 300 blackout – it’ll take larger calibers.

What is 458 SOCOM good for?

The 458 SOCOM was originally designed for use with Delta Force in Mogadishu (see/read Blackhawk Down). The combatants were used to chewing a drug known as khat that would increase users pain tolerances. This increased paint tolerance gave way to more rounds needed to kill combatants. The round’s bullet is so large that it was meant to put someone down on the first shot, whereas the factory loads 5.56mm rounds would pass through the body but the combatant would continue attacking.

Hog hunting in 458 SOCOM anyone?

While considering the 458 SOCOM, the military also considered a 7.62 round, and the 450 Bushmaster. The 7.62 was dismissed because it would need a totally new rifle platform to shoot. The 450 Bushmaster was ruled out because the bolt required for it had proprietary steel properties that were owned by General Motors – so the military did not want to get in the middle of that.

Take the 458 SOCOM on your next big game hunting adventure

What is the best home defense AR?

I wouldn’t recommend a 458 SOCOM for your home defense round. In a home defense situation, this would definitely give you the stopping power. However it might go right through your target, into another room. The sheet bullet weight on this round is sure to make an intruder give pause.

This is where an EPC-9 would come in. It won’t go through walls (or more than one at least) and will do the job so you and your family can escape.

What caliber AR-15 is the best?

While we feel it depends on the individual to decide which caliber is best, we would like to walk you through some potential points you should consider when building your AR-15.

Learn about Aero Precision’s M4E1 and X15 AR15 Platforms here.

Availability of cheap ammo

Will you be able to find ammunition to shoot your rifle? In October of 2021, the answer is yes. Will you pay Trump era prices for it? Heck no. As a gun dealer I’m here to tell you that buying factory ammo at a fair price is impossible. So selling it at a fair price is even more impossible.

The MAGA AR-15 rifle cartridge

Worth Firearms sells factory ammunition in 5.56 NATO ammunition at prices below your local range. Grab it while its in stock!

It used to be that a gun owner could buy imported ammunition from Russia (usually steel cased versus typical brass cased). The Biden administration however has banned all imported Russian ammunition (see the NRA’s article here).

No more AR-15 steel old Russian round or other cartridges from Russia

This doesn’t mean that finding ammunition is out of the question – you’re just going to pay an arm and a leg on it. That goes for most calibers too. The AR-15 platform can be converted to a 9mm rifle capable of shooting factory loads (see Who Makes the Best 9mm AR Pistol) and people have caught on – but now 9mm ammunition is hard to find!

Reloading ammunition at home

Don’t even get me started on hand reloading ammunition at home. The knowledge barrier to entry is very high. If you know someone who personally does this – ask them to teach you how. I have a friend (a fellow FFL) that was heavily into reloading. He even bought reloading devices to make his output faster.

However, once factory ammo became scarce – many people took to reloading from home. That caused components (bullets, gunpowder, casings, primers) to become scarce as well.

Unique calibers = unique difficulties

If you build your AR-15 to shoot the unique calibers mentioned above (basically anything besides 9mm, 223/5.56, or .300 blackout), you’re setting yourself up for issues from the get-go. Factory ammo in these calibers are hard to come by. Even worse – you might need to reload your own ammunition.

Reloading ammunition can be a pleasing and fun pastime. You don’t really get the reloader’s mindset until you yourself have “policed” your own brass from a range. If you’re at the range and you see people squatting down looking at used casings, it’s highly likely they’re a reloader. And they’re looking for free casings that you’d typically either throw away, recycle, or keep on the ground.

You’ll also need to find components. Components for these unique calibers may be in better supply today, but it’ll still cost up front for bullets, gunpowder, and primers.

Long range shooting or short range shooting

You’ll need to consider if you’ll shoot short , intermediate, or long range. Short range can be categorized as zero to one hundred yards. Intermediate could be categorized between one hundred to three hundred yards (in my opinion it would be as long as five hundred yards). Long range shooting would be beyond five hundred yards.

Short distance

Your 9mm AR carbine will not have an effective range beyond 100 yards. It’s best suited for those shorter distance ranges or close engagements. Aero Precision is working on a .45 caliber EPC-9 rifle – more to come on that. However, I think an AR-15 in 9mm would be great except there are a few issues. A 9mm AR-15 will not hold the bolt open after a magazine is empty. There may be workarounds that I’m unfamiliar with. The EPC-9 from Aero Precision has this feature and might be your best bet if you’re going the custom built 9mm AR carbine route.

Aero Precision EPC-9 - Short distances and short barrels

Send us an e-mail at brad@worthfirearms.com to get your EPC-9 built by us! These make excellent home defense firearms.

Intermediate distance

This is where your 223/5.56 and .300 blackout comes into play. Both are great rounds for this distance. My time in the Marine Corps was spent at the 200 – 500 range distances. You’ll get plenty of use out of these calibers at these ranges. The .300 blackout’s effective range extends just short of 500 yards. The bullet weight for the .300 blackout heavier than the 223.

Worth Firearms stocks the M&P 15 and Aero Precision custom AR-15 builds for this category. We can also special order any other brand you might want.

Long range shooting

Your main choice here would be the .223/5.56 caliber rifle. People have been said to shoot up to 750 yards accurately with these calibers. If you’re target shooting, give this a go. If you’re defending our nation against all enemies both foreign and domestic, you might want to stick with a 500 – 600 yard range at max.

Long ranges = Longer Barrels??

Rifle availability

Who wants to buy a rifle they cannot even get? That question doesn’t make sense – but it’s true. There is definitely some supply chain issues surrounding manufacturing in general. It’s even worse in the gun industry. Hording, labor shortages, anti-2A rules/laws, and many other issues have put pressure on manufacturers.

You may want a Daniel Defense MK12 but it isn’t in stock at big distributors or their website. Perhaps you’re better off buying a custom build? Or a clone build? Think about your options here. There are ways to outsmart the availability issue.

You could also go to GunBroker.com and find one there. Be prepared for the sticker shock though.

The 3 best AR-15 calibers

Worth Firearms will rate the 3 best AR-15 calibers based on availability and cost. If you read everything up to this article, I’m sure you can guess which ones we’re choosing.

Read: Best Barrels for an AR-15 Build

#1. .223 Remington/5.56 NATO – Most versatile cartridge

Really this shouldn’t come as a surprise to anyone. Think about the availability of ammunition. The fact that it’s the standard cartridge for our US military. The parts and accessories surrounding the .223 REM/5.56 NATO are boundless. From barrels, lower receivers/upper receivers, bolt carrier groups, charging handles, it’s all made with this caliber in mind.

We sell the parts to make your own .223 REM/5.56 NATO rifle.

#2. .300 AAC Blackout

We personally have not shot one of these but people rave about them constantly. Telling us and taking to the message boards how fun it is to shoot it. These rifles’ effective ranges are just about 500 yards. They make a great second choice in your AR-15 platform’s caliber lineup.

#3. 9mm – Aero Precision Pistol Caliber EPC-9

That’s it – that’s the gun. But it’s not a true AR-15? That’s true – but if you’re planning to shoot a 9mm round (cheaper than the others below), then you might as well buy a gun that’s going to do it right.

But I want to switch uppers between 5.56/223, 9mm, and .300 blackout! That’s a good reason too – except to use a 9mm magazine, you’ll need to install a magwell accessory to seat the magazine properly.


What caliber is an AR-15?

We’ve done a deep dive into the various calibers for you – from large to small, unique to mass produced, domestic and foreign to give you an idea of what caliber you should build your AR-15 into. The choice is yours based on how you’ll use your AR platform rifle.

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