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What Calibers Can Be Used in AR15 Lower: 5.56 and More

L.p. Brezny | Updated February 23, 2024 | Why You Should Trust Us | How We Earn Money
various bullets for AR15 rifles

When it comes to popular guns, the AR-15 is pretty much number one in the United States, with massive sales over the last decade or so. Part of the appeal is that the rifle can fire so many different calibers. Let’s look at them all so you know what you can shoot with this versatile weapon.

What Are The Best AR-15 Lowers Out There?

If you’re curious as to which we have a full article on the the different types of AR lowers on the market today. We explain which ones are worth it and which to avoid.

What Calibers Can Be Used in AR15 Lower?

Man holding standard AR rifle

There are two primary pieces of an AR-15 rifle – the upper and the lower. The upper contains the barrel and firing mechanism, while the lower contains the magazine, trigger, and handle. (1)

When discussing which calibers can be used in an AR-15 lower, you have to consider the upper you’re using too.

That said, here is a pretty complete list of calibers that can be used with a standard 5.56 NATO AR-15 lower.

Calibers Compatible With Standard AR-15 5.56mm Lower

  • .204 Ruger
  • .223 Remington
  • .22 Nosler
  • .224 Valkyrie
  • .25-45 Sharps
  • 6.5 Grendel
  • 6.8 SPC
  • .300 Blackout
  • .300 Ham’r
  • .30 Remington AR
  • .350 Legend
  • .450 Bushmaster

As you can see, there are tons of options available, which is partly why so many gun owners have an AR-15 – they can shoot pretty much anything they want.

What Calibers are Interchangeable?

For calibers to be interchangeable, they must be the same size and loaded at similar pressures. As a rule, you can always size down, but you can never size up. (2)

For an AR-15 lower, the calibers that are interchangeable include:

  • 223 Remington
  • 22 Nosler
  • 224 Valkyrie
  • 6 mm ARC
  • 6.5 Grendel
  • 6.8 SPC
  • 300 Blackout
  • 300 HAMR
  • 30 Remington AR
  • 7.62 x 39 rounds
  • 350 Legend
  • 450 Bushmaster
  • 458 SOCOM

For more reading see our article on the different bullet sizes and how far can a 5.56 bullet travel.


Typical AR with a few different magazines

Can you use any lower for any caliber?

When it comes to the AR-15, you can use any caliber that measures 5.56 or below with any AR-15 lower. If a caliber is larger than 5.56, it won’t fit or fire correctly. All lower sections are basically compatible with all uppers, so you can interchange them at will. (3)

What caliber can a AR-15 lower use?

There are around 30 calibers compatible with an AR-15 lower section, so you have a vast selection of different bullets you can use. Any caliber that fits a 5.56 or below should work inside an AR-15, which is partly why so many options are available. Primary examples include 204 Ruger, 223 Remington, 6.5 Grendel, 300 Blackout, and 30 Remington AR.

Can an AR lower use any caliber?

If you’re using a Multi AR-15 lower, it can fire any caliber that fits a 5.56 or 223 designation. So, any bullet that isn’t larger than 5.56 should fit and work with a Multi AR lower.

What calibers can be used on a 5.56 lower?

There are far too many calibers that can be used on a 5.56 lower to mention them all here. However, with over 30 varieties, some of the most popular calibers that can be used on this lower include 17 Mach 2, 22 Long Rifle, 17 Remington, 204 Ruger, 243 Winchester Super Short Magnum, and a 350 Legend.

Can you put a 6.5 Creedmoor upper on a 5.56 lower?

No, you cannot put a 6.5 Creedmoor upper on a 5.56 lower. The Creedmoor is designed for an AR-10 rifle, not an AR-15, so the two parts are not compatible with each other. When building an AR-15, you have to make sure the upper and lower sections fit perfectly for the gun to work.

Can a AR-15 shoot 223 and 556?

If you have a Multi AR-15 lower, it can shoot both 223 and 556 ammunition. Even if it’s just listed as a 5.56 lower, it’s also compatible with 223 caliber rounds. However, if your AR-15 is chambered for 223 only, you can’t shoot 5.56 ammunition.

Why can’t you use 556 in a 223?

You can’t use 5.56 caliber rounds in a 223-chambered rifle because 5.56 bullets are loaded with more pressure, and a 223 rifle isn’t built to withstand that kind of pressure. So, even though the bullets are identical in size, they’re not the same inside. 

Can you use 7.62 in an AR?

Yes, you can fire a 7.62 caliber round in an AR-15 if you have the correct upper. You can mount a 7.62 upper to a standard 5.56 lower, but you will also have to swap the magazine to accommodate the larger and heavier bullets.

Can you use a 5.56 lower for 450 Bushmaster?

Yes, you can mount a standard 5.56 NATO AR-15 lower to a 450 Bushmaster upper. This setup is ideal if you’re big-game hunting and want to be able to fire the massive Bushmaster rounds without having to buy a completely separate gun.

Can you use an AR-15 lower for 9mm?

Yes, you can use an AR-15 lower with 9mm rounds, but to do so, you have to install a conversion block. Because 9mm bullets are smaller than the standard rounds for an AR-15, a conversion block makes the two pieces compatible. As a rule, you can always go smaller with your rounds, but never bigger than what the gun is rated for.

Does .300 Blackout and 556 use the same lower?

Yes, a 300 Blackout and a 556 can use the same lower – they’re interchangeable. The bullets used in both pieces are almost identical in size and shape, and the amount of pressure is negligible, meaning you can fire Blackout rounds without risk to the gun or yourself.

Can you use an AR-15 lower for 7.62 x39?

Yes, you can use an AR-15 lower for a 7.62 x 39 upper. Many shooters like this setup because it allows them to use AK-47 rounds in an AR-15. Since 7.62 rounds are heavier and have more stopping power, they’re fun for target practice.

Can you put a 6.5 Creedmoor on AR-15 lower?

No, you cannot put a 6.5 Creedmoor on a standard AR-15 lower because the Creedmoor is built for an AR-10 rifle. While it’s theoretically possible to rig the gun to fit both pieces together, it takes too much effort and could wind up damaging both sections.

Is a 308 and 6.5 Creedmoor the same lower?

Technically, the 308 and the 6.5 Creedmoor are not the same lower, but they are compatible with each other, so you can use them interchangeably. That said, you’ll need to use a high-pressure bolt and firing pin if using a 6.5 Creedmoor on an AR-15 lower. 

Can you shoot 6.5 Grendel in AR-15?

Yes, you can shoot a 6.5 Grendel in a standard 5.56 NATO AR-15 rifle. This ammunition is similar enough in size and pressure to shoot just as effectively as a 5.56 or 223 round.


  1. Jake Bleiberg and Stephanie Dazio, Design of AR-15 could derail charges to popular rifle. Retrieved from https://apnews.com/article/nv-state-wire-us-news-ap-top-news-ca-state-wire-oh-state-wire-396bbedbf4963a28bda99e7793ee6366
  2. Merriam Webster, caliber. Retrieved from https://www.merriam-webster.com/dictionary/caliber
  3. Annette Doerr, WHAT IS AN AR LOWER RECEIVER? Retrieved from https://www.guns.com/news/what-is-an-ar-lower-receiver

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