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

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

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.

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.

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…

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?  

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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

What’s your take on .300 BLK ARs?

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.

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.

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.)

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.

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!

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.

Additionally, you’ll need a 7.62x39mm BCG. The rim is much larger than a 5.56 round, so you gotta step it up.

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.

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.

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.

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. 

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.

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.

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.

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.

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!