AR-15 Upper Receivers – Best 5 Calibers to Own ~ Video

What is an AR-15 Upper Receiver?

Changing an AR-15 upper receiver alters the original firearm in one of several ways. Since the upper receiver contains the barrel, bolt carrier group, and the receiver itself, the possibilities are almost endless. Shooters can change the caliber, barrel length, and even operating method of the firearm by simply replacing the upper receiver.  Depending on the caliber, this may require using a different magazine.

Three AR-15 uppers - 7.62x39mm, .300blk and 5.56mm
Three AR-15 Uppers in 3 different calibers, but they use the same lower. IMG Jim Grant

U.S.A.-( AR-15 rifles, carbines, and pistols are extremely modular, making buying upper receivers so popular. The good news is that these are extremely easy to work on and install and normally don’t require any special tools. Simply push out the takedown and pivot pins and remove the upper receiver – it contains the barrel and the matching bolt carrier group, the charging handle, and the gas system.

If you’ve zeroed an optic or scope, it will not hold zero when you switch between uppers, as normally, this results in a shift in the fired round’s point of impact. So you’ll likely want to have a dedicated optic for each upper, but once you have that squared away, just replace the original upper assembly with a different one; push the two pins back into place, and voila! You’ve got a different gun.

That said, there are a few limitations. Standard 5.56mm lowers can’t accept larger .308 uppers, and vice versa. But there are plenty of AR 15 upper receivers in all sorts of calibers that will work perfectly with standard 5.56 lowers, and the lineup seems to expand all the time. Many complete upper assemblies are available that are literally “plug and play” on any standard AR lower assembly.

And if you’re comfortable with a few basic tools – like a vise and a barrel nut wrench – you can easily build up your own stable of extra AR-15 complete upper receiver assemblies. Quality suppliers like Brownells have all the barrels, uppers, handguards, bolt carrier groups, and associated tools you’d need.

Why would anyone go to the trouble of getting a bunch of uppers to go on just one AR-15 lower?

Magpul BAD Lever
Magpul BAD Lever

Think of it this way. You’ve spent time and money getting exactly the trigger you want and the best stock that fits you perfectly. Maybe you’ve added a Magpul BAD Lever and the ideal pistol grip for your hand. Why should you give all that up just because you want to do a different kind of shooting at a different distance?

The good news is that you don’t have to, at least not if you get a spare complete upper assembly or three. That way, you can use the same gun for everything from plinking soda cans on the family farm, to sniping steel targets way out there, to hammering feral hogs in thick brush, just by pushing two pins and swapping uppers.

5 Most Versatile AR-15 Upper Receiver Calibers

  • 458 SOCOM
  • 6.5 Grendel
  • 300 AAC Blackout
  • 22 LR
  • 6mm ARC

I’m basing this list on the assumption that you’ve already got an AR-15, most likely a 16-inch carbine in 5.56. And if you don’t yet, now is the time to buy, as there are tons of bargains out there on 16-inch carbine style ARs in 5.56.

But here’s my list of the Top 5 AR-15 Uppers every Black Rifle shooter should think about owning.

5) 458 SOCOM Upper Receiver:

458 SOCOM Ar-15 Upper
458 SOCOM Big Bore Upper

As good as the 5.56 cartridge is, there are some things it just doesn’t do that well. Like delivering ethically potent hits on big, tough game animals like feral hogs. But .458 SOCOM isn’t the only option available, there are several other big bore cartridges that fit through an AR-15 mag well. Better yet, it seems like more are developed by creative wildcatters every year.

Some of the really popular big-bore AR cartridges include .450 Bushmaster, .458 SOCOM, and .50 Beowulf. All of them throw big fat bullets at relatively slow velocities and are devastating at close range. If you want readily available factory ammo, .450 Bushmaster might be the best bet. If you handload, .458 SOCOM probably gives you the most versatility, as it uses rifle bullets available in a wide range of weights and styles – as heavy as 600 grains if you want.

And if you really must have a caliber that starts with a 5, and named after an epic hero, then .50 Beowulf is the only choice.

Cross Machine Tool Co - AR-15 458 SOCOM Billet Upper Receiver
Cross Machine Tool Co – AR-15 458 SOCOM Billet Upper Receiver

Perhaps the best aspect of adding a Big Bore AR-15 upper receiver is that you can typically use standard 5.56 magazines, although Bushmaster does add a special “single stack” follower and springs to standard 20-rd magazines specifically for use with .450 Bushmaster ammo. Any of these cartridges will give your AR-15 performance similar to the venerated 45-70 at close range, in a lightweight, semi-auto package. The possibilities are endless.

If you decide to build your own Big Bore AR-15 upper receiver assembly, be sure to either enlarge the ejection port or get an upper with an ejection port specially made to accommodate the big fat cartridge cases.

4) 6.5 Grendel Upper Receiver

6.5 Grendel Ammunition
6.5 Grendel Ammunition: 6.5mm Grendel round showing a variety of bullets 144 gr (9.3 g) to 90 gr (5.8 g)

The 6.5 Grendel is an amazing cartridge. It uses 6.5mm bullets, known for having good sectional density and relatively high ballistic coefficients – which means they fly very well, even in stiff crosswinds. The Grendel gives performance similar to, if not better in some ways as the .308, but in a cartridge that can fit inside a standard AR-15 lower.

C-Products AR-15 6.5 Grendel Magazine
C-Products AR-15 6.5 Grendel Magazine

Just a few years ago, the 6.5 Grendel’s creator, Bill Alexander, opened the door for even more manufacturers to crank out ammo, parts, and barrels with the name “Grendel” on them.

Not surprisingly, Alexander also first developed the .50 Beowulf and took the Grendel’s name from the same Old English saga.

Unlike the Big Bore uppers, a 6.5 Grendel will require a magazine specially shaped for that round. For reaching way out there with a standard-sized AR-15, it’s hard to beat the 6.5 Grendel.

Be sure to add a good-quality scope and some nice rings to complete the package.

3) 300 AAC Blackout Upper Receiver

300blk AR-15 pistol LaRue Tactical
This .300blk AR-15 pistol from LaRue Tactical uses the same AR-15 lower as any other gun. IMG Jim Grant

The 300 AAC Blackout cartridge gives performance very similar to the 7.62×39, but in a cartridge that works perfectly with standard AR-15 bolts, magazines, and lowers. You can convert any standard 5.56 AR-15 to .300 AAC Blackout by merely changing only the barrel. Of course, if you don’t want to mess with taking apart your current upper, you can just drop a complete 300 AAC Blackout AR 15 upper receivers onto your lower.

Why not go with an AR-15 upper in 7.62×39 in the first place? You can use less-expensive steel-cased ammo in a 7.62×39, but there are tradeoffs. Typically, you’ll also need to put in an extra-power hammer spring to help reliably ignite the sometimes-harder primers found on cheap, foreign-made steel-cased ammo. Many shooters also add in harder firing pins to help set off primers in less-expensive steel-cased ammo.

7.62x39mm AR-15 SBR
7.62x39mm AR-15’s like this SBR utilize a proprietary magazine but still use a standard AR-15 lower. IMG Jim Grant

Finally, you must have specially-shaped mags for a 7.62×39 AR. But the 300 AAC Blackout uses the standard 5.56 mags that you probably already have in good supply.

Where the 300 AAC Blackout really shines is with sub-sonic loads combined with a suppressor. When suppressed, 300 blackout is remarkably quiet and has little felt recoil. Despite this, it lets you thump targets with bullets weighing up to 220 grains. If you live in a state that allows suppressor ownership, you really should consider getting an upper in .300 AAC Blackout with a suppressor-ready muzzle device.

2) 22 LR AR-15 Conversion Kits

Yes, I know we’re still in a .22 LR drought, but there seem to be signs of it easing. Maybe. Even with .22 LR hard to come by in some locales, a dedicated .22 LR upper makes a lot of sense for AR-15 owners.

You can get conversion kits that let you shoot .22 LR through any standard 5.56 AR-15, just by swapping out the bolt and carrier group and using a special magazine. And these kits are almost always less expensive compared to dedicated .22 LR AR 15 Conversion Kits. Many shooters get these kits and enjoy them a lot. But there are some reasons why you might want to consider a dedicated .22 LR upper.

First, dedicated .22 LR uppers typically have barrels specifically rifled to work with slower, lighter, all-lead projectiles fired from rimfire cartridges – something like a 1-16 twist. The twist rate in your 5.56 AR-15’s barrel is intended for 55 grain, or heavier, jacketed bullets moving at much faster speeds. The slower twist rate often results in better accuracy from a dedicated .22 LR upper compared to a conversion kit.

Second, true .22 LR uppers are blowback, and don’t need a gas block or gas tube, and are thus often lighter. Your 5.56 AR still wears its gas block and gas system parts when you swap out just a bolt carrier group and magazine to convert to .22 LR.

More accurate and lighter easily translate into more fun, which are both important, especially if you’re using your .22 LR upper to help introduce young or new shooters to the game.

1) 6mm ARC Upper Receiver

CMMG Endeavor 6mm ARC – its upper receiver still works with a standard AR-15 lower.

The newest conversion kit caliber to hit the market is also one of the best ways to achieve long-range capability with the AR-15 platform. The 6mm ARC was recently developed by Hornady and provides better performance at longer ranges than the .223/5.56 and functions reliably from an AR-15. With standard FMJ bullets weighing in at 108gr, the 6mm ARC will deliver more energy on target than the .223/5.56 and still be comparable to the energy found in the larger 308.

.223 (Left), 6mm ARC (Center), .308 (Right)
.223 (Left), 6mm ARC (Center), .308 (Right)

All you need to convert an AR-15 to a 6mm ARC rifle is a 6mm ARC barrel, a 6.5 Grendel bolt, and a 6.5 magazine; that’s it! Once that is assembled you will have in your hands a lightweight rifle chambered in a mid-sized cartridge that is easily capable of reaching targets at 1,000 yards. The 6mm ARC has been reviewed in depth by AmmoLand, you can read more here or watch our video below.

New Take On Old Saw

There’s an old cliché out there that goes something like this, “Beware the man with only one gun. He probably knows how to use it.”

But if that “one gun” is an AR-15, it can quickly turn into a whole lot of different guns, just by pushing out two little pins, slapping on a new AR 15 Conversion Kits assembly, and pushing those two little pins back into place.

About Jim Grant

Jim is one of the elite editors for, who in addition to his mastery of prose, can wield a camera with expert finesse. He loves anything and everything guns but holds firearms from the Cold War in a special place in his heart.

When he’s not reviewing guns or shooting for fun and competition, Jim can be found hiking and hunting with his wife, son, and their dog Peanut in the South Carolina low country.

Jim Grant

Disclosure: Some of the links below are affiliate links, meaning at no additional cost to you, Ammoland will earn a commission if you click through and make a purchase.
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“For reaching way out there with a standard-sized AR-15, it’s hard to beat the 6.5 Grendel.” Depends on what one expects to be done once the bullet is “way out there.” Missed the two best newbbies on the block….224 Valkyrie and 6mmARC. Try them, or the Grendel, or actually any other calibers, in a side charge straight pull bolt action version, sans gas system (gas block, tube, key, rings) for true free float barrel (barrel custom ordered without gas port), no action thrashing, no dirty gas into the action, no mass on the barrel. With an equal quality barrel, this… Read more »

Last edited 1 year ago by StLPro2A

Except for the .22, I’d say buy a complete rifle. Save up the few hundred dollars more Going for a 6.5mm makes sense if you hunt big game, but I still think spending the extra money on a complete rifle is worth doing.

Xaun Loc

This article covers the one and only reason to consider .300BO: if you are in a state that allows suppressors and want to run subsonic ammo in a suppressed AR and are willing to put up with the NFA limitations on your suppressor. BUT the article omits one significant reason why you should never consider .300BO as an conversion upper for your current 5.56 AR. Actually I shouldn’t really say that the article omit it – actually the article mentions it, but pretends it is an advantage: “the 300 AAC Blackout uses the standard 5.56 mags that you probably already… Read more »


we have run 300 black AND 556 uppers at the range and in the field. never any problem with mixing up the mags or the ammo.
a little common sense goes. a long way.


Good for you and your luck. It really is a common mistake.


Be warned that when adopting uppers from different cos their measurements for the pin placement may be off enough that it won’t really fit the lower! How do I know? I have an upper I’ll sell cheap!


but…. buttt…. they’re all mil spec, right?


Did not see one good caliber! And that Magpul SAD GET U KILLED Lever


I’d say this list has some redundancy in it. Points 1 and 4 should/could be combined. A 16″ 5.56 is plenty accurate at range for what 5.56 is capable of provided you have a good barrel. If you really want precision gun you might as well go with 6.5 Grendel or Creedmoor. Points 5 and 3 should/could be combined. 300BLK is more than capable for feral hogs (you could make the argument that 5.56 is sufficient). The only reason you should go with .450, .458 or .50 Beo is if you are taking Elk or you want a meme gun.… Read more »

Last edited 1 year ago by WiffleballTony
Deplorable Bill

I agree. A good 16″ (or less) in 556 is good for close. A good, accurate 20″ (or longer) in 556 will work for medium range to say 600 yds. Anything past that, the grendel shines past 1,000 yds. 458 is good for elk, buffalo, elephant, semi trucks, helo’s etc. A good anti material round. Blackout only shines when it’s in pistol form, suppressed with heavy bullets. If you want a 30 round 30-30 get a 762 x 39, or a 762 x 40 WT or a 300 hammer. There is also now the 350 Legend. That’s a really good… Read more »


6.5mm Creedmore requires a .308 lower.