Best AR-15 Calibers & Conversion Kits: 5.56 No More

We love the AR-15 platform because the possibilities are endless, especially with an AR-15 caliber conversion kit.

Want to swap in a new trigger, grip, bolt carrier group, or barrel? You can! All it takes is basic tools and a little know-how. Lots of people build their own AR-15 from scratch -- it's that easy.

Changing your AR's caliber is actually pretty simple, and we have lots of great recommendations for you.

Read on to learn more!

Comparison Chart

How We Tested the Best AR-15 Caliber Conversion Kits

For this article, we gathered a bunch of AR-15 caliber conversion kits we've personally shot, reviewed, and owned here at Pew Pew Tactical. The entire Pew Pew Tactical team, as well as freelance writers, contributed to this review to offer different opinions.

For guns we have reviews on, we've linked those so you can read more. Every review handgun undergoes a strict testing protocol that requires a minimum of 500 rounds of both FMJ and JHP ammo through it at 5 and 7 yards.

Meanwhile, every review rifle undergoes 500 rounds of practice ammo and heavier match-grade ammo at 50 and 100 yards with a 5-25x scope (unless otherwise noted). We require all AR-15s to be tested with at least 3 magazines to assess compatibility, and we use a Lyman Digital Trigger Gauge to measure the average trigger pull for all guns.

Best AR-15 Caliber Conversion Kits

1. Palmetto State Armory .300 BLK Upper Receivers - Best .300 BLK Conversion Kit

Best .300 BLK

PSA .300 BLK Upper Receivers

$187

at Palmetto State Armory

Prices accurate at time of writing

$187 at Palmetto State Armory

Specs:

• Caliber: .300 BLK
• Barrel Length: 7-16"
• Handguard: M-LOK, KeyMod
• New Bolt Required: No
• Magazine Adapter Required: No

There's no easier AR-15 caliber conversion than .300 Blackout. As usual, Palmetto State Armory has carbine- and pistol-length ARs for everyone.

This cartridge is specifically for suppressing and subsonic rounds are widely available.

A .300 Blackout upper will use the same bolt carrier group, magazines, buffer, and more.

Converting to .300 BLK is easy, but it's so similar to 5.56 NATO that there are some safety concerns. It's easy to get the two mixed up if you aren't careful.

While you can use standard 5.56 magazines with .300 BLK, you may want to invest in dedicated .300 Blackout magazines. Either way, mark them so you don't grab a mag full of the wrong ammo and blow up your AR.

And for more, check out our guide to the Best .300 BLK Uppers.

Federal American Eagle .300 BLK – 500 Rounds

$519

at Ammunition Depot

Prices accurate at time of writing

$519 at Ammunition Depot

What's your take on .300 BLK ARs?

2. CMMG .22 LR Bravo - Best .22 LR Conversion Kit

Best .22 LR

CMMG 22 LR Bravo Conversion Kit

$164

at Brownells

Prices accurate at time of writing

$164 at Brownells

Palmetto State Armory (see price)

Primary Arms (see price)

CMMG (see price)

Specs:

• Caliber: .22 LR
• Barrel Length: N/A
• Handguard: N/A
• New Bolt Required: Yes (included)
• Magazine Adapter Required: No (magazine included)

If you want to save money on your live-fire training, there's nothing better than shooting .22 LR. It's practically free!

You can buy a dedicated .22 LR upper, but let’s keep things as affordable as we can. Who's trying to spend a ton of money to save some money?

A .22 LR conversion is the cheapest conversion you can do to a 5.56 NATO AR-15. It allows you to shoot budget-friendly ammo that's perfect for practicing skills and training new shooters.

Because the .22 LR is nearly the exact same diameter as 5.56, you don’t need to swap your barrel, upper, gas system, magazine well, or anything crazy like that. All you need to do is swap out the bolt carrier group.

The CMMG system offers a built-in buffer system and replaces your standard BCG. It comes with a single magazine, and spares are readily available.

As a one-piece system, the CMMG bolt is perfect for quick swaps and range trips. The CMMG bolt works surprisingly well, and I enjoy it.

The magazines are affordable, but I never needed more than two for my shooting.

There's only one downside: .22 LR ammo is very dirty. The stainless steel bolt wipes clean easily enough, but you'll have more cleaning to do than you would with 5.56 NATO.

The conversion makes the gun a blowback design so it's reliable in both direct impingement and gas piston guns.

CCI Stinger .22 LR

$7

at Palmetto State Armory

Prices accurate at time of writing

$7 at Palmetto State Armory

Brownells (see price)

Lucky Gunner (see price)

Check out more of our favorite AR-15s in .22 LR!

3. Faxon Firearms Bantam - Best 9mm Conversion Kit

Best 9mm Conversion Kit

Faxon Bantam 16” Complete 9mm Upper Receiver

$455

at OpticsPlanet

Prices accurate at time of writing

$455 at OpticsPlanet

Faxon (see price)

BattleHawk Armory (see price)

Specs:

• Caliber: 9mm
• Barrel Length: 16"
• Handguard: M-LOK
• New Bolt Required: Yes
• Magazine Adapter Required: Yes

Without a doubt, one of the most popular conversions for an AR-15 is 9mm. It's the most popular centerfire pistol round and the Faxon Firearms Bantam is a great complete upper for your AR-15.

As a PCC you can use it in most indoor ranges, as well as compete in USPSA PCC division. 9mm ammo is pretty cheap (outside of ammo shortages) and a bit more fun to shoot than a rimfire round.

A 9mm AR doesn’t run off the same direct-impingement or piston operating method as a 5.56. Instead, most utilize a direct blowback-style operating system. This makes them simple and relatively affordable.

You will have to modify your lower receiver to shoot 9mm, but it's not difficult and you can easily reverse these changes.

Stern Defense makes magazine well adapters for Glock, S&W M&P, Beretta 92, and SIG P320 magazines.

Best Mag Coversion Kit

Stern Defense 9mm Conversion Adapter

$179

at Brownells

Prices accurate at time of writing

$179 at Brownells

OpticsPlanet (see price)

Rainier Arms (see price)

You should also look into a heavy buffer and buffer spring to keep your AR-15 cycling smoothly.

Check out our CMMG Bantam PCC review and even more of the Best 9mm Uppers if you want to get into the world of pistol caliber carbines.

AAC 9mm 115 Grain FMJ 100 Round Box

$25

at Palmetto State Armory

Prices accurate at time of writing

$25 at Palmetto State Armory

4. Radical Firearms 7.62x39 Upper Receiver - Best 7.62x39mm Conversion Kit

Best 7.62x39

Radical Firearms 7.62×39 Upper

$239

at OpticsPlanet

Prices accurate at time of writing

$239 at OpticsPlanet

Specs:

• Caliber: 7.62x39
• Barrel Length: 16"
• Handguard: M-LOK
• New Bolt Required: Yes (included)
• Magazine Adapter Required: No (magazines required)

While the AR is nearly the polar opposite of the AK in terms of rifle design and philosophy, it's pretty easy to convert your American AR into an instant Vodka fan with this Radical Firearms 7.62x39 upper.

The 7.62x39 cartridge is an excellent round for inside of 300-meters (remember, we're on the metric system this time). Inside an AR-15, it’s decently accurate and a real close-quarters hammer.

Check out our Best 7.62x39 Ammo guide for some of our faves.

For a long time, it was tough to convert an AR into 7.62x39 and make them run reliably, but these problems are mostly smoothed out now.

This complete upper is ready to rock. All you need are some new magazines. The 30 rounders are straight goofy but the 20 rounders look a bit more normal. You want to go for quality with these magazines so opt for C-Products and D&H.

AR-15 D&H 7.62×39 Magazine

$24

at Gunmag Warehouse

Prices accurate at time of writing

$24 at Gunmag Warehouse

Lastly, you may or may not need some trigger work.

The primers in Russian ammo are often quite hard, so you may need an extra power hammer spring from Wolf to make sure they keep kicking off.

Experiment first, and then see if you need a little more oomph.

Hornady American Gunner 123-Grain 7.62×39

$73

at Ammunition Depot

Prices accurate at time of writing

$73 at Ammunition Depot

5. Palmetto State Armory .224 Valkyrie Upper Receiver - Best .224 Valkyrie Conversion Kit

Best .224 Valkyrie

Palmetto State Armory .224 Valkyrie Upper Receiver

$399

at Palmetto State Armory

Prices accurate at time of writing

$399 at Palmetto State Armory

Specs:

• Caliber: .224 Valkyrie
• Barrel Length: 20"
• Handguard: M-LOK
• New Bolt Required: Yes (included)
• Magazine Adapter Required: No (magazines required)

We've had a ton of shooting .224 Valkyrie long-range, and Palmetto State Armory makes a great upper for budget-conscious shooters.

We’ve talked about .224 Valkyrie quite a bit here at Pew Pew Tactical.

It’s a short-action round designed for long-action performance. Best of all it uses parts that are all already in circulation for the fabled (and failed) 6.8 SPC.

This upper from PSA is all about going the distance. It comes with a heavy-contour stainless steel barrel that will stay cool and on target longer than your run-of-the-mill AR-15. Stick a bipod under this thing with the M-LOK rail and get ready to shoot further than you ever did with 5.56 NATO.

LWRC 6.8 SPC Magazine

$24

at Gunmag Warehouse

Prices accurate at time of writing

$24 at Gunmag Warehouse

Brownells (see price)

Some 5.56 magazines will work, but 6.8 SPC magazines are pretty affordable and accessible to find.

If you build your own, you’ll need the parts listed above and a 6.8 bolt, but you can use a standard bolt carrier group.

The .224 Valkyrie is a bit Gucci right now so expect to pay a pretty penny for the parts and the ammunition. It’s still a fun round that’s extremely accurate, but also soft-shooting and fun.

Hornady Varmint Express .224 Valkyrie

$28

at Ammunition Depot

Prices accurate at time of writing

$28 at Ammunition Depot

Read our full PSA .224 valkyrie review for all the info!

6. Bear Creek Arsenal .17 HMR Upper Receiver - Best .17 HMR Conversion Kit

Best .17 HMR Conversion Kit

Bear Creek Arsenal .17 HMR Upper Receiver

$269

at Bear Creek Arsenal

Prices accurate at time of writing

$269 at Bear Creek Arsenal

Specs:

• Caliber: .17 HMR
• Barrel Length: 20"
• Handguard: M-LOK
• New Bolt Required: Yes (included)
• Magazine Adapter Required: No (magazines required)

I love .17 HMR. It’s super accurate, surprisingly powerful, and easily one of the best rounds for taking small game and even medium-sized predators -- especially with this upper from Bear Creek Arsenal.

You can make your AR a .17 HMR tack driver through one of the coolest conversions ever. This upper has a heavy-contour barrel that will happily put tiny .17-caliber rounds on target all day long.

Of course, you'll need a new magazine.

I own a .17 HMR AR-15 and love it.

Hornady .17 HMR Varmint Express V-MAX

$14

at Brownells

Prices accurate at time of writing

$14 at Brownells

Palmetto State Armory (see price)

Lucky Gunner (see price)

7. Bushmaster .450 Bushmaster Upper Receiver - Best .450 Bushmaster Conversion Kit

Best .450 Bushmaster

Bushmaster Bravo Zulu .450 Bushmaster

$589

at GrabAGun

Prices accurate at time of writing

$589 at GrabAGun

Specs:

• Caliber: .450 Bushmaster
• Barrel Length: 16", 20"
• Handguard: M-LOK
• New Bolt Required: Yes (included)
• Magazine Adapter Required: No (follower or magazine required)

If you just wish your AR-15 had way, way more stopping power, this Bushmaster conversion kit has your name written all over it.

The .450 Bushmaster cartridge is a hoss, but it fits inside AR-15 magazines and works with standard lower receivers. Might want to invest in a tough optic, and maybe some ice packs, because the recoil takes a toll on optics and shoulders alike.

Since Bushmaster developed it for the AR-15 platform, converting is as easy as dropping the supper onto your current lower.

Hornady Black .450 Bushmaster 250 gr FTX

$37

at Palmetto State Armory

Prices accurate at time of writing

$37 at Palmetto State Armory

Sportsman's Warehouse (see price)

Optics Planet (see price)

If you need a sledgehammer of a hunting rifle or just like slinging big chunks of lead (nothing wrong with that), step up to the plate and give this fella a swing.

How to Pick the Best AR-15 Caliber Conversion Kit

Why would I want to convert my AR-15? Why not just buy an entirely new rifle?

First of all, it's cheaper. You also don't have to go through an FFL to make your purchase since it's the lower receiver that the ATF considers a firearm; the upper receiver is just an assembly of parts.

And shooting more calibers is good, clean fun!

I convert between different calibers based on my use case. I swap to .22 LR when I’m bored or want to train new shooters and run .17 HMR when I hunt rabbits or coyotes.

You might have a feral hog problem that calls for something heavier than 5.56 NATO. Maybe you want to train on mute with subsonic .300 BLK and a suppressor. Perhaps you have a bunch of 9mm ammo and want to train with that instead of more expensive rifle ammo.

They're all valid reasons to convert your AR-15 to a new caliber!

The key is to identify your needs, find a suitable cartridge, and look for a conversion kit that fits the bill.

Meet the Experts

Pew Pew Tactical writer Travis Pike authored this article. Travis spent a lifetime shooting as a kid and later joined the United States Marine Corps, where he spent five years as an infantryman. In the middle of his Marine Corps career, he began writing and never stopped. Travis is an NRA-certified instructor and Concealed Carry Instructor for the state of Florida. He has thousands of articles to his name with a variety of publications, including Pew Pew Tactical.

Editing this article is Scott Murdock. Scott is a Marine Corps veteran who competed and qualified as a rifle and pistol expert while in service. In addition to shooting, Scott has written for a variety of publications, testing, researching, and evaluating guns and gear. He brings that knowledge and skillset to this article, editing and fact-checking for accuracy.

Editor-in-Chief Jacki Billings runs our experienced team of reviewers. She is a National Rifle Association Basic Pistol Instructor as well as a member of the Society of Professional Journalists, ACES: Society for Editing, and the Professional Outdoor Media Association. Jacki has a bachelor’s degree in journalism and has worked as a media professional for close to 20 years, specializing in gun media for almost 10 years. With 2,000+ articles to her name, she uses her professional journalism and editing experience to set testing protocols and editorial standards for Pew Pew Tactical.

Final Thoughts

The AR-15 is a brilliant design and a testament to that is just how far people are pushing it.

Its ability to swap calibers is fantastic and useful for those that want one rifle that can basically do it all.

Have you guys done any converting? If so, what? Let us know below. For more great AR-15 content, take a look at our Definitive AR-15 Guide!

Latest Updates

April 2025: Added the .450 Bushmaster conversion kit. Removed the Brownells 9mm adapter and Colt magazine combo.