Best AR-15 Calibers & Conversion Kits [2019]

No two ARs are the same.

You can swap almost every single part for something different. The AR-15 is an exciting piece of firearms history and as time has passed its only become a better weapon.

Transformer gif
An AR can’t transform that much…yet!

An AR can be made for close quarters battle with a 7.5-inch barrel, or long-range precision with a match grade 20-inch rifle. You can swap grips, triggers, stocks and more. One other thing you can do is easily convert your AR to a variety of calibers.

The majority of AR-15s come in the classic 5.56 NATO round. This little fellow was what the AR-15 was designed to function with and is often the most affordable option for new AR owners.

That being said you may have a different caliber AR-15, but more than likely you’ll be able to convert your AR to a wide variety of calibers.

Mostly what we have to start with is a traditional Multi Cal lower receiver.

aero receiver set
Aero Precision receiver set, note the Cal Multi

This is your typical AR-15 lower receiver that is typically used with a 5.56 caliber AR-15.

AR lower receivers that are designed to take Glock magazines or something like that cannot be converted to a multitude of different calibers. 

PSA PX-9 Magazine Release
PSA PX-9 for Glock magazines

Why Would You Want to Convert?

Now Travis, why would I ever want to convert my AR-15? Why swap uppers, buffers and more? Why not just buy an entirely new rifle?  

Best AR-15 Buttstocks
You can never have too many lowers!

Those are all solid questions, and you are indeed welcome to do that, but there are many great reasons to use a base gun to swap calibers. The entire state of California is one, along with many other states that impose fees and ridiculous waiting periods to buy a gun.

In other situations, it’s all about keeping that one perfect lower receiver.

The majority of your controls are in the lower receiver. That 200 dollars adjustable trigger can be used for more than one caliber, as can that sweet B5 stock, and Magpul BAD lever, and those ambi safeties and magazine releases.

A stock lower isn’t hard to walk away from, but that lower you’ve tossed some real money in it will be an excellent host for a number of conversions.

Hangover Math Gif
Geissele trigger + PRS stock + LaRue grip + titanium anti-walk pins…

In my case, I convert between three different calibers based on use. I switch my beater PSA rifle with some conversions that I don’t use all the time. I swap to .22 LR when I’m just bored or want to train new shooters or convert to .17 HMR when I hunt rabbits or coyotes.

I don’t shoot these calibers often enough to have a reason to own a dedicated rifle in them.

Your reasons are your own, and I’m just here to guide you through the process.

We are going to talk about the most popular calibers to convert your gun too, as well as what you’ll need to convert your rifle to that particular caliber. These aren’t the only calibers or methods of conversion, but these should get you started!

9mm

Without a doubt, one of the most popular conversions for a AR-15 is taking it and converting it to 9mm.

9mm is the most popular centerfire pistol round, and it’s an excellent round for the AR-15. A 9mm AR doesn’t run off the same gas system as a 5.56 and is a blowback style weapon.

PSA 9mm Upper
PSA 9mm Upper

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

And it makes for a cheap day at the range!

We’ll need a few things:

  • 9mm Upper (Or barrel and the willingness to swap barrels.)
  • 9mm BCG – These BCGs are designed for Colt SMG magazines, Glock magazines, or a hybrid. A hybrid is the easiest to find and will work fine.
  • Heavy Buffer and Heavy Buffer Spring

Magazines and Magazine Conversion Kit.

Stern Defense MAG AD9 has multiple models and you can use Glock magazines, S&W M&P magazines, Beretta 92, and SIG P320 magazines in your AR 15.

Best 9mm AR-15 Conversion
190
at Brownells

Prices accurate at time of writing

Prices accurate at time of writing

Brownells also offers a magazine well conversion block that is designed to use Colt SMG magazines. Colt magazines are incredibly durable and well made. They are a classic choice for Colt SMG builds, and the Brownells design is robust but expensive.

200
at Brownells

Prices accurate at time of writing

Prices accurate at time of writing

Magazine conversion kits are designed to allow your gun to feed 9mm in a multi-caliber receiver. A magazine designed for a 9mm round is a lot smaller than a 5.56 magazine so you’ll need the means to feed your gun. 9mm is so popular that multiple options exist.

However, we haven’t found one that is reliable and functions across a wide enough variety of builds.

Check out even more of our Favorite 9mm Uppers if you want to get into the world of PCC (Pistol Caliber Carbines).

.22LR

.22LR is a very cheap round to shoot and easily the most economical conversion you can do for your AR-15 rifle.

You can go with a dedicated upper, but let’s go as affordable as we can. Who is trying to spend a ton of money to save some money?

A .22LR conversion is the cheapest conversion you can do to a 5.56 caliber rifle. It allows you to shoot cheap ammo and is perfect for training new shooters and practicing your skills on a budget.

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

Well, it’s not just the bolt, but it’s a one piece drop in conversion. This one-piece system is from CMMG and has a built-in buffer system and replaces your standard BCG. The kit 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 2 for my shooting.

Best AR-15 .22 LR Conversion Kit
220
at Brownells

Prices accurate at time of writing

Prices accurate at time of writing

The CMMG bolt is incredibly well built and is made from high-grade stainless steel. .22LR ammo is very dirty, and the stainless steel bolt wipes clean easily enough. The conversion makes the gun a blowback design so its reliable in both DI and gas piston guns.

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

.300 Blackout

.300 Blackout is the little round that could. Every year it seems we get some new wonder round that will replace the 5.56 as the standard AR caliber rifle.

Every year this pretty much fails.

.300 Blackout never promised to replace the 5.56, and it would just compliment it.

Wilson combat Suppressed .300 Blackout
Wilson Combat Suppressed .300 Blackout

The .300 Blackout has been quite successful and why not?

It’s a great round to run an SBR or pistol on because of its excellent short barrel performance. Let’s not forget the fact it’s perfect for suppressing with subsonic rounds, and best of all it’s a straightforward conversion to make.

The .300 Blackout uses the same BCG, magazines, buffer, and more. All you need is a new barrel or just a new upper. You can use the same BCG and lower without modification as well as the same magazines.

250
at Palmetto State Armory

Prices accurate at time of writing

Prices accurate at time of writing

A quick look at uppers shows you can get an affordable one for less than 250 bucks from Radical Firearms. While you can use standard 5.56 magazines, you may want to invest in dedicated .300 Blackout magazines.

This is a safety concern because mixing up the ammo is dangerous. Plus the .300 Blackout magazines from Magpul will provide smoother feeding. The magazine both looks and feels different than a standard 5.56 magazine.

This used to be a rifle. Used to be.
This used to be a rifle. Used to be.

The .300 Blackout is an excellent conversion if you want to move towards a suppressed option.

What’s your take on .300 BLK ARs?

Readers' Ratings

4.61/5 (97)

Your Rating?

And for more…check out our Favorite .300 BLK Uppers.

7.62x39mm

Rifle is fine! The 7.62 x39 is a classic Russian round that has remained popular due to its use in AK rifles.

3 Upgraded AK-47s
3 Upgraded AK-47s

While the AR is nearly the polar opposite in terms of rifle design and philosophy of the AK series of rifles.

But if you really want to take advantage of the beefier 7.62×39 cartridge, it’s pretty easy for you to convert your American AR into an instant Vodka fan.

Tested AK 7.62x39 Rounds
Tested AK 7.62×39 Rounds

The 7.62 Russian round is an excellent round for inside of 300 meters. Inside an AR it’s decently accurate and a real close quarters hammer. Plus, it’s cheap, really cheap ammo.  Check out our Best 7.62×39 Ammo post.

It’s fun to shoot and is an excellent round for hunting in states with minimum sized caliber requirements.

For a long time, it was tough to convert an AR into 7.62 Russian and it was hard to make them run reliably, but these problems are primarily smoothed out now. The uppers need good M4 feed ramps, and they need to be polished.

The PSA KS-47 series of uppers are quite reliable and come in a variety of configurations, including pistol and rifle.  It’s great in that it takes AK magazines in an AR platform.

But, they require a proprietary lower – so it isn’t a true conversion…but it is a great rifle!

KS-47 Fit and Finish
KS-47 Fit and Finish

Check out our full review of the KS-47.

Radical firearms have a very affordable series of uppers too that might be worth taking a look at.

240
at Primary Arms

Prices accurate at time of writing

Prices accurate at time of writing

Additionally, you’ll need a 7.62x39mm BCG. The rim is much larger than a 5.56 round, so you are going to need to step it up. Brownells is producing a full auto BCG in 7.62 x39 and they are very high quality!

80
at Brownells

Prices accurate at time of writing

Prices accurate at time of writing

This conversion requires a new proprietary magazine with a crazy weird curve to it. The 30 rounders are straight goofy. The 20 rounders look a bit more normal. You want to go for quality with these magazines, and I only know two companies that produce magazines that work for the AR and this caliber.

These two come from C-Products and D&H. These two companies produce some great magazines and allow you to run your gun reliably.

20
at Brownells

Prices accurate at time of writing

Prices accurate at time of writing

Lastly, you may be a bit pigeonholed when it comes to triggers and this gun. 7.62 x 39’s primers are often quite hard, a signature of Russian ammo. So you may need an extra power hammer spring from Wolff to make sure they keep kicking off.

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

.224 Valkyrie

The .224 Valkyrie is the newest wonder round, and we’ve talked about it quite a bit here at Pew Pew Tactical.

The .224 Valkyrie has been a success, and I bet it will be a success because like the .300 Blackout it’s an easy caliber to convert your 5.56 gun into.

Federal American Eagle 90 gr vs 75 gr .224 Valkyrie
Federal American Eagle 90 gr vs 75 gr .224 Valkyrie

.224 Valkyrie is a long range round, and likely the most effective longest range round you can shove into a AR 15. It’s a short action round designed for long action performance. Best of all it uses parts that all already in circulation for the fabled (and failed) 6.8 SPC.

You can quickly build a .224 upper with a standard receiver and .224 barrel, handguard, etc.

Of course, you can buy a complete .224 Valkyrie upper, and this may be the best bet if you want precision. Pros have the tools, time and experience to build a tight, hard running upper receiver.

Best Long-Range AR-15 Budget Upper
499
at Palmetto State Armory

Prices accurate at time of writing

Prices accurate at time of writing

Cross Machine and Tool makes a very sleek and modern upper with barrels ranging from 20 to 24 inches long. This upper comes with a BCG that’s ready to roll as well.

PSA has you covered with a budget too with a 20-inch barrel and a BCG and charging handle. 

150
at Palmetto State Armory

Prices accurate at time of writing

Prices accurate at time of writing

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

C Products makes them from 5 rounds up to 28. LWRC makes a modern polymer magazine called the SIX8, and it can hold 30 rounds and features a transparent window. Both magazines are affordable, and they work reliably.

23
at Brownells

Prices accurate at time of writing

Prices accurate at time of writing

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.

.17 HMR

I love the .17 HMR round. It’s super accurate, powerful, and easily one of the best rounds for taking small game and even medium-sized predators. The .17 HMR round is a fun round, but it’s traditionally a bolt action rifle round.

.17 HMR Vs. .22 LR
.17 HMR Vs. .22 LR

You can make your AR a .17 HMR tack driver through one of the coolest conversions ever. To do so, you need an entirely new complete upper receiver as well as a different spring an add on to your buffer. You’ll, of course, need a new magazine.

Good news is a company called Garrow produces it all. You can order a complete kit from them, and it comes with everything you need to run .17 HMR.

Garrow 17HMR Upper
Garrow 17HMR Upper

What’s even cooler is that this isn’t a blowback design, it’s a gas operated, locking bolt, semi-auto blaster. It’s a delayed blowback system that incorporates the delayed blowback system into the bolt and receiver walls. It uses ball bearing to delay the bolt from opening.  

There are lots of reasons why this is practical and safe for a rimfire round, but to me, it’s just gun geek cool.

I own one and love it. It’s a very slick tack driver with a stainless steel barrel, it’s threaded for a suppressor and two high-quality magazines are tossed into the system. You can find it at Garrow Development, and I highly recommend it.

Best Hunting .17 HMR
12.75
at Lucky Gunner

Prices accurate at time of writing

Prices accurate at time of writing

Calibers and Conversions

The AR-15 is a brilliant design and a testament to that is just how far people are pushing it. The ability to swap calibers is fantastic, and as far as I know, no other weapon can convert like the AR-15.

This isn’t even an exhaustive list, just a list of the most popular and common. We have 410 ARs, 458 SOCOM, 6.5, 6.8 SPC, various pistol calibers and more. If it exists in a relatively short action, you can throw it in an AR.

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!

13 Leave a Reply

  • Allen Walker

    6.5 Grendel Man,6.5 Grendel.

    3 months ago
  • Dennis

    .243 WSSM in 55 grain will run 4170 FPS and is perfectly suited for the short action AR platform. Heavier bullets will anchor a deer at extremely long ranges.

    3 months ago
  • dwest

    Beowulf ? ?

    3 months ago
  • Travis Dugan

    I just can’t believe you left out the 6.5 Grendel. Arguably the best option regarding the article. However, mistakes happen and I know you’ll get it right next time.

    3 months ago
    • Pogo

      I thought the same thing. 1 sentence at the end lumps together the popular calibers.

      3 months ago
  • Brandon

    6.8 SPCII has a tighter chamber and turns your AR into a sweet little deer rifle. 358 Yeti (wildcat) is another caliber using cut down .308 brass and sends a 200 gr pill downrange at 2400+ fps. Bit more of a kick but it drops anything it hits. Details on FB.

    3 months ago
    • Flyingdog

      Plus a fantastic hog killer. The caliber of choice for feral hogs.

      3 months ago
  • PriklyPete

    Excellent article, I love the AR platform and it has infinite variables.

    3 months ago
  • J.R.

    I can't believe you would leave out the 6.5 Grendel! An amazing round that will take out medium size game with low recoil, excellent distance and maintains good fps out of a 12" barrel It is on fire at our gun club currently most purchased caliber. What gives?

    3 months ago
    • Fast eddy

      22 noseler is a good round a barrel change and 6.8 magazines gets you down rang fast and far good for target and critters,ammos hard to fing and expensive but if you have 556&black out its another option.

      3 months ago
  • Pogo

    To my knowledge the PSA KS-47 7.62x39 upper requires a PSA KS lower. You can’t just swap it onto a standard AR lower and be good to go.

    3 months ago
    • CheesyPotato

      "But, they require a proprietary lower – so it isn’t a true conversion" The author covered that.

      3 months ago
      • Pogo

        Why put it in at all???? It’s a platform onto itself and is no conversion at all. It doesn’t fit with the subject matter. Might as well mention mutants which have been around much longer or S&W’s dedicated 22LR AR. The popular calibers were barely mentioned as an afterthought. 17HMR?? Probably the least popular conversion done. This being said this article was 1000% better than the “humor” articles.

        3 months ago
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