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

The AR-15 is an exciting piece of firearms history and a versatile weapon.

Modded AR-15s
You can pretty much customize AR-15s to what you want.

It can be made for close-quarters battle with a 7.5-inch barrel, or long-range precision with a match-grade 20-inch barrel.

You can swap grips, triggers, stocks, and more. It’s like the transformer of the gun world.

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

Another thing you can do easily is convert your AR to a variety of calibers.

Though a majority of AR-15s come in the classic 5.56 NATO round, with a conversion kit you can swap out to another caliber.

Faxon Firearms Ascent AR-15
Faxon Firearms Ascent AR-15

So today let’s walk through AR-15 conversion kits. We’ll look at why you might want one and give you some recommendations.

Keep reading…

Table of Contents

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Why Would You Want to Convert?

Now, why would I 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!

You are indeed welcome to do that, but there are many great reasons to use a base gun to swap calibers.

States that impose fees and ridiculous waiting periods to buy a gun often make things more difficult. A conversion setup makes things a bit easier.

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

Aero Precision Thunder Ranch TR15 Lower
Aero Precision Thunder Ranch TR15 Lower

That $200 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 a lower you’ve tossed some real money into 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 different calibers based on use — .22 LR when I’m bored or want to train new shooters .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.

Colt M4 in .22 LR
Colt M4 in .22 LR

With that out of the way, let’s talk calibers.

9mm Conversions

Without a doubt, one of the most popular conversions for an AR-15 is 9mm.

It’s the most popular centerfire pistol round, and 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 (outside of ammo shortages) 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).

.22 LR Conversions

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

Because the .22 LR 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 need to do is swap out the bolt.

5.56 NATO, left, .22 LR, right.

Well, it’s not just the bolt, but a one-piece drop in conversion. The system, from CMMG, 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.

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

Prices accurate at time of writing

Prices accurate at time of writing

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

The CMMG bolt is incredibly well built and is made from high-grade stainless steel.

.22 LR ammo is very dirty, and the stainless steel bolt wipes clean easily enough. The conversion makes the gun a blowback design so it’s reliable in both DI and gas piston guns.

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

.300 Blackout Conversions

.300 Blackout is a great round to run an SBR or pistol with because of its excellent short barrel performance.

It’s also perfect for suppressing with subsonic rounds. Best of all, it’s a straightforward conversion. The .300 Blackout uses the same BCG, magazines, buffer, and more.

All you need is a new barrel or 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 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

4.92/5 (568)

Your Rating?

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

7.62x39mm Conversions

The 7.62 x39 is a classic Russian round that’s 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, if you really want to take advantage of the beefier 7.62×39 cartridge, it’s pretty easy to convert your American AR into an instant Vodka fan.

Popular 7.62x39 Ammo
Popular 7.62×39 Ammo

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.

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

(Check out our Best 7.62×39 Ammo for some of our faves.)

7.62x39mm
7.62x39mm

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.

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

And 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!

BEST BANG-FOR-THE-BUCK 7.62x39 AR Rifle
700
at Palmetto State Armory

Prices accurate at time of writing

Prices accurate at time of writing

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.

379
at OpticsPlanet

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 gotta step it up.

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 so opt for 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.

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 Wolf to make sure they keep kicking off.

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

.224 Valkyrie Conversions

.224 Valkyrie is a long-range round, and likely the most effective longest-range round you can shove into an AR-15.

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.

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

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

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 five rounds up to 28. LWRC makes a modern polymer magazine called the SIX8, and it holds 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 Conversions

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 Round
.17 HMR Round

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.

What’s even cooler is that this isn’t a blowback design, it’s a gas-operated, locking bolt, semi-auto blaster. A delayed blowback system that incorporates the delayed blowback system into the bolt and receiver walls. It uses a 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, threaded for a suppressor and with two high-quality magazines. I highly recommend it.

Best Hunting .17 HMR
19
at Palmetto State Armory

Prices accurate at time of writing

Prices accurate at time of writing

Conclusion

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.

Aero AR-15 10.5" Pistol
Aero AR-15 10.5″ Pistol

Worth mentioning, this isn’t an exhaustive list as .410, .458 SOCOM, 6.5, 6.8 SPC, and other calibers exist for the AR-15. Basically, 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!

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32 Leave a Reply

  • Commenter Avatar
    RP

    I was hoping to see more options for hunting? Why aren't the other calibers discussed here (i.e.6.5)?

    November 13, 2021 11:16 am
  • Commenter Avatar
    RP

    I was hoping to see more options for hunting? Why aren't the other calibers discussed here (i.e.6.5)?

    November 3, 2021 2:03 pm
  • Commenter Avatar
    sound awake

    i have 3 x ar 15s in 7.62x39
    2 x 16.5 carbines and 1 x 10.5 pistol
    i use mil spec hammer springs enhanced firing pins and 28 round c products magazines in all of them
    theyre stone cold reliable with tula ammo
    and accurate too
    the one carbine recently put 5 tula fmjs in a group just under two inches at 200
    yards with a primary arms 1-4x arms on top and a psa ept trigger in the lower
    all 3 uppers are from bear creek arsenal
    250 bucks shipped includes nitrided bcg mlok rail and cnc billet upper receiver

    November 1, 2021 5:35 pm
  • Commenter Avatar
    Gerald San Agustin

    no 0.50 Beowulf?

    November 1, 2021 10:50 am
  • Commenter Avatar
    James F Gordon

    I converted to 300 Black out.also 450 Bushmaster,Both give great accuracy. I really like the 450 for Deer and Hogs.I only use 5.56 for varmints or just for fun at the Range to practice my trigger pull and safe gun loading and accuracy. The AR is a great platform to Build on.

    November 1, 2021 6:36 am
  • Commenter Avatar
    Mark Campbell

    After an unimpressive experience with an M16 in 1975, I wasn't interested in AR's for a long time. My prairie dog hunting {and other nefarious activities} buddy kept after me. My favorite phrase of his on the subject was "you know, it's been 40 years". There's a great story behind how I finally made the actual purchase, but I'll save that for another time.
    After having used my Ruger SR15 VT on a couple trips to the dogtowns, and loving it very much, my buddy told me about the Sharps Rifle Company .25-45 Sharps. For the price of the upper, I could change to a caliber that among other things, would be legal for deer in MN, where I live. Another huge advantage in my mind was the ability to make brass from 5.56 NATO or .223 REM brass. But at the time there was no load data for the .25-45 Sharps!
    Being a stickler for, um, just about everything, I buckled down for the process of working up my own load data. I've reloaded for years, and so was confident in the fact that I better keep my healthy paranoia going or I'd end up with a really nice new upper that was totally useless and funny looking to boot.
    I started looking at other calibers made from 5.56/.223 cases, 6mm & so forth. I checked case capacities and load data, and finally loaded up 5 loads from 23.5 grains to 26 grains of powder, 5 rounds of each. Then I shot them at 100 yards, looking for pressure signs all along the way. The groups went from fairly open to smaller & smaller, with the smallest being at 25 grains, and then at 25.5 grains it started to open up again. I confirmed this over & over, and 25 grains of that powder is what I load to this day. And that's the same powder charge I use for 5.56/.223, very handy.
    I've always liked the smallest load needed to accomplish the task. They are more accurate, cheaper to load, and easier on equipment, and they allow for a safety margin. Working up that load for my first AR conversion is the most fun I've had reloading, and possibly shooting too, in my entire life!!!

    May 18, 2021 9:42 pm
  • Commenter Avatar
    Rob

    Got 300 blackout and a CMMG 22lr conversions. Love them both. Neither one is a tack driver. But since the 300 is a CQC self defense weapon and the 22lr is just plain fun to shoot I don't need that type of accuracy

    May 18, 2021 8:47 pm
  • Commenter Avatar
    Russ

    I need one place that I can get a complete ar9mm converson upper,big,mag block. No piece meal.

    February 27, 2021 3:22 pm
  • Commenter Avatar
    Todd

    No .458 socom?!

    August 2, 2020 11:21 am
  • Commenter Avatar
    Robert Mitchell jr

    Looking for a AR 410, upper receiver?

    June 3, 2020 2:49 pm
  • Commenter Avatar
    Angus

    Garrow....what happened to them? I looked them up, nothing available plus a lot of unhappy customers who claim to have paid for their orders and never received the product?

    April 29, 2020 3:47 am
  • Commenter Avatar
    Mark

    CMMG .22LR conversion, in an AR with a properly sighted 1-6x optic.
    What is the likely impact change, vs 5.56/.223 - 55/62gr on 50yds and under targets?

    April 23, 2020 8:59 am
    • Commenter Avatar
      David, PPT Editor

      Probably minimal, but you should test to make sure.

      April 23, 2020 9:19 am
    • Commenter Avatar
      no CMMG

      The CMMG kit sucks ass. Much better off having a separate 22 like the 10/22 with upgraded trigger.
      I tried the CMMG kit and it did not work in my AR. I have a custom trigger in the AR which may be the problem with the fail to fires. Doesn't matter what type of 22 lr ammo used. The custom trigger is 2.5 lbs.

      May 2, 2021 2:29 pm
      • Commenter Avatar
        David, PPT Editor

        Many lightweight triggers have issues with light striking CMMG kits. That isn't really the fault of the CMMG kit and is just one of the design limitations that you have to work with.

        May 2, 2021 3:00 pm
  • Commenter Avatar
    LC

    I thought this was going to be an article on the QCB (quick change barrel kits) available. I'm in the process of doing T&E of three or more of them. I'm not interested in the Pantheon Arms Dolos.

    For 9x19mm, you pretty much need a separate upper as it uses a different BCG. For .223 Wylde, you can use a QCB to convert it to anything that uses a standard 5.56mm NATO BCG and do .22 LR with the CMMG .22 LR conversion kit which gives you .300 BLK. I've pretty much decided I want the KAK Industry 6¼" Baby Blond barrel. However, I want to try the RightToBear 6" and V Seven 6½" barrels. I think around 6" is the sweet spot for .300 BLK AR pistol barrel lengths especially if you're going to run suppressed.

    I've pretty much decided to standardize on Aero Precision uppers.

    October 14, 2019 11:18 pm
  • Commenter Avatar
    Evan H

    Is there a reason the 204 Ruger dint make your list? It is a high velocity, highly accurate round that is a necked down 223 casing. Just curious, as you have a laundry list of calibers that require more than just an upper swap. I mean some of these your changing out to different mags or converting the mag well and having to swap out spring rates to allow for proper cycling of the AR without damaging it.

    Just curious.

    September 12, 2019 8:33 am
    • Commenter Avatar
      David, PPT Editor

      No reason against it! .204 Ruger is just a fairly niche cartridge and uppers can be a bit hard to find. It will definitely be looked at when we update this article though ;)

      September 12, 2019 11:39 am
  • Commenter Avatar
    Allen Walker

    6.5 Grendel Man,6.5 Grendel.

    April 29, 2019 8:47 am
    • Commenter Avatar
      Steve Canaan

      Beat me to it.

      October 31, 2021 5:06 pm
  • Commenter Avatar
    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.

    April 29, 2019 8:06 am
  • Commenter Avatar
    dwest

    Beowulf ? ?

    April 29, 2019 3:51 am
  • Commenter Avatar
    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.

    April 29, 2019 2:33 am
    • Commenter Avatar
      Pogo

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

      April 29, 2019 7:36 am
  • Commenter Avatar
    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.

    April 28, 2019 8:13 pm
    • Commenter Avatar
      Flyingdog

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

      April 29, 2019 9:00 am
  • Commenter Avatar
    PriklyPete

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

    April 28, 2019 7:54 pm
  • Commenter Avatar
    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?

    April 28, 2019 5:20 pm
    • Commenter Avatar
      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.

      April 28, 2019 9:03 pm
  • Commenter Avatar
    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.

    April 28, 2019 3:09 pm
    • Commenter Avatar
      CheesyPotato

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

      April 28, 2019 5:18 pm
      • Commenter Avatar
        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.

        April 29, 2019 7:30 am
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