Don’t get overwhelmed with the huge selection of cartridges that can be used in the AR – what do you really want to accomplish with the gun?
The AR-15 is a special gun. You can make it yours with a few basic changes and usually without any tools, provided you swap a fully integrated upper receiver kit onto the top of a complete lower receiver. Even if you opt for more labor-intensive modifications, the fact that the modular design and the huge market for components and accessories exists, makes building the perfect AR an easy task generally.
This article is about one of the single most foundational changes that can be made to the AR to affect its capabilities immediately and comprehensively – Changing the Caliber.
So, let’s get into it.
It can feel a bit daunting, since there is a plethora of technical information that goes into the different distinctions – here’s a slightly simpler, basic guide
Caliber is kind of a funny word – it really refers to the diameter of a projectile, but it’s been morphed into a catch-all phrase that can refer to the Cartridge, or even more granularly, the specific load that is being shot out of a firearm. Caliber still technically references a single thing only, but in the industry, we’ve come to embrace the idea that caliber and cartridge at the very least can be used sort of interchangeably.
In the case of the AR-15, it’s an important distinction because natively the gun shoots a .223 Remington or a 5.56x45mm (they are not back and forth compatible – more on that later), which are “.22 Caliber”. But these .22 Calibers are vastly different from the .22LR that you grew up shooting out of your Ruger 10-22 or Marlin 60.
The .30 Caliber .308 Winchester, which is basically the same cartridge as the 7.62x51mm is also an “AR Platform” favorite (the AR10 or .308 AR pattern). And that cartridge is vastly different from another .30 caliber cartridge – the .300BLK which uses a .223 cartridge case but a .30 caliber bullet. The .300BLK fits in a standard AR-15 format (magazine well and action length), but the .308 and some other .30 caliber cartridges only fit in the larger, heavier AR-10/.308 platform.
Again, a bit confusing, but here’s what you need to know:
- Caliber does not exactly equate to cartridge, even if people throw the terms in for each other occasionally.
- Not all cartridges will be similar, just because they shoot a projectile in the same class (caliber) as another cartridge.
- Some cartridges on the AR-15 will require a different bolt (and carrier), because they use a case that is not the same as a .223/5.56; others will require a completely different platform from the AR-15 because they are too long to fit in the magwell, or be handled by the action length of the native AR-15 upper receiver.
- ALL cartridge changes require a barrel, with very, very few exceptions.
- .223 and 5.56 are basically the same thing, except they aren’t – you can shoot a .223 Remington out of a 5.56 barrel, but you cannot shoot a .5.56 out of a .223 barrel. You can shoot either or both out of a .223 Wylde barrel. They use the same bolts and carriers, generally speaking. The 5.56×45 is a higher-pressure round.
- Additionally, 7.62x51mm and .308 Winchester are the same thing, except they aren’t. You can shoot a 7.62x51mm out of a .308 Winchester barrel, but not generally the other way around. They use the same bolt and carriers. The .308 is a higher-pressure round.
- There are some mainstream cartridges that are meant to fire out of the AR-15; and separately there are some cartridges that require the AR-10 or .308 AR pattern of receivers.
Some more information about this will be in each below section where it applies.
- What’s the purpose of your AR-15?
- General Sport Shooting Is My Goal
- I want a Home Defense Gun That Gives Me The Upper Hand
- I Want to Make Tiny Groups on Paper Targets and Show Them Off
- I Need a Gun That Can Do More Than Just One Thing
- What Are The Basic Numbers When It Comes To The Different Calibers?
- Wrapping Up the Caliber Argument For the AR Platform
What’s the purpose of your AR-15?
Seeing as this is an article about the AR-15, we will mostly focus on that, but the reader should understand that there are new, hot, heavily marketed cartridges that may be aimed at the AR consumer, that don’t even fit in the AR-15. So, we will try to keep it as simple as possible and state which of these cartridges fit in which platform, where needed.
Even still, the way most shooters will decide which cartridge they want to use, is dependent on what they want to do or accomplish with their rifle/carbine/pistol build of the AR family of guns. Below we listed some mainstream concepts and talk about the applicability of the different mainstream cartridge or caliber options that are particularly well suited for those activities. If you skip to the section, you are most interested in you can find the basics about well-matched cartridges and calibers for those types of activities. We also recommend reading the whole article to get a good feel for the basics across the board, so you feel more knowledgeable about the variances.
General Sport Shooting Is My Goal
If you just want a gun to go to the range with and have a budget that isn’t built with a specialization in mind, you will likely be a good candidate for the native cartridge offerings for the AR-15. These cartridges are the 5.56×45, the military (NATO) cartridge for the platform; and the .223 Remington, the “civilian” round for the platform in layman’s terms. A deeper dive in the history might only complicate things at this point, so suffice it to say, they are both substantially similar in today’s AR-15 landscape.
Here are some reasons to choose the .223 Remington or the 5.56x45mm cartridge for your AR-15:
- Ammunition is plentiful – even in times of market scarcity, you can still generally find some available, or the manufacturers will prioritize their runs of these calibers over others
- Recoil is very manageable
- Reliability is proven
- Accessories for this “vanilla” version of the AR abound, so it won’t be “vanilla” for very long
- Performance for the cartridge is excellent across the board, and it’s cheap enough for many people to shoot in bulk
- It could also be used secondarily as a varmint or medium game hunting firearm, or could be used as a home defense weapon – the US Military, after all, uses it for warfighting
- Accuracy is more than acceptable, and even low-end builds will usually still be able to do a 2 MOA group or better under most circumstances to 100 or 150 yards (roughly equivalent to a 2.25 inch or better 5 shot group at those distances)
- It’s very easy to live in the 5.56/.223 AR ecosystem – it’s been around for a very long time and in high demand for more than a decade
Hunting With an AR Is My Jam
If you really need to find a great sporting rifle for hunting big game, the AR-15 can be perfect for the intermediate game like deer, from 115 lbs. to 450 lbs. Or for anything smaller, depending on what cartridge you choose. In full transparency, with the right loads and the right build, the .223/5.56 rifles will be very good for this hunting regimen too. There is basically nothing in the under 450 lbs. size range, and in the less than 300 yards distance range that a .223/5.56 cannot dispatch in the field, if the right load and variables are considered.
But there is also .300 BLK
The .300 BLK is a purpose-built product that uses a .223 case that necks up into a .30 caliber projectile, which allows for subsonic delivery of a larger caliber bullet than the AR usually handles. This means it is very easy to use a suppressor with and has the economies of scale when it comes to sourcing brass and components. It can also be run as a supersonic load, that allows more distance, a larger wound channel on bigger targets, and a much heavier grain weight bullet. These variables can play out in one of two ways:
.300 BLK in a suppressed hunting round can be used on larger deer sized game at ranges out to about 175 yards, and suppressed, it is very quiet (it used to be called the 300 Whisper – the Blackout is the more marketable name that was funded as a larger project by AAC, a Remington subsidiary at the time; ultimately the SAAMI specification came this way). You can expect to take bigger animals in closer ranges with a lot of added quality of life features, and more than adequate terminal ballistics for those close-up ranges. The problem is that you need to be careful as you extend the range because there begins a slippery slope of large bullet drop as you move out further from the intended target.
.300 BLK in a supersonic non-suppressed deliverable is basically a less powerful version of the .308 with less recoil and less terminal ballistics and range. It still hits hard, offers a lot of grain weight, but only delivers great performance to about double that of the suppressed version – so probably on that same deer sized game, it will get you out to 350 yards with great terminality. The powder charge behind the larger projectile is a bit less potent, so it doesn’t have the staying power of other .30 caliber cartridges, but in normal AR hunting ranges it can definitely do the job.
You’ll see comparative basic ballistic numbers down below in a dedicated section so you can compare all the cartridges side by side.
And 6.5 Grendel
The 6.5 Grendel is where AR hunting really finds its stride, but it’s almost overkill for what anyone would realistically need for intermediate game in North America. The balance between longer ranges, higher power delivered on target; a larger grain weight projectile, and the speed at impact make it a very good cartridge for deer sized animals as a test case. It’s small enough for coyote, but still overkill, but likely way too big for most normal varmint hunting.
You get screaming fast projectiles, with decent wound channels and excellent accuracy. Like tack driving accuracy. Like – too much accuracy for the ranges listed above. In fact, you are probably good to go with a 6.5 Grendel further out than 650 yards, easily, which is much further than most people would be hunting with an AR anyway. A 6.5 Grendel remains supersonic past 1200 yards. You can’t even expect to be hunting with most ARs at 1200 yards. The point? You’re good to go for that deer on the next ridge with the right load and variables, and you can expect to get your shot right on target. Good luck packing out your animal.
And 6.8 SPC
The 6.8 SPC is quite similar to the 6.5 Grendel, with some specialty rounds it is excellent for deer sized targets, even if it is meant for military purposes. It’s going to deliver .308-like performance out to the intermediate ranges of 600 yards and is suitable for 400 lb. Targets of all types.
You will see it delivers good grain weight and energy dump on target and there are few who complain about the added utility of the 6.8SPC, or the 6.5 Grendel for that matter. The performance is pretty close, so they often can be used in an either/or fashion based on what you need. You’re going to get slightly enhanced energy on target (closer in) from the 6.8SPC and enhanced accuracy from the 6.5 Grendel, generally.
And the .224 Valkyrie
The .224 Valkyrie is a cartridge that even compares favorably with the 6.5 Grendel thanks to some basic dimensional characteristics. It’s harder to find components and make a build, but if you value the extra bit of oomph provided by the energy retention, and the slight velocity and range benefits over the 6.5 Grendel, and don’t mind the smaller projectile, this is a fantastic round for longer range shots against a wider range of the small and intermediate game spectrum.
Basically because of the enhancements in performance, you are getting a cartridge that completely bridges the prairie dog to big deer game range about as well as any other round on the market. You’re also sort of bridging the gap between 6.5 Grendel and 6.5 Creedmoor, though it’s got a bit more drift and drop than the Creedmoor at 850-1050 yards, depending on a few variables.
It’s a great round if you need a single gun to bridge the gap, and you need longer ranges, without having to take too many big game species.
I want a Home Defense Gun That Gives Me The Upper Hand
You can count on the same Cartridges listed above. If a round can easily dispatch a 450 lb. animal with a single shot at distance, it’s going to be fine going against a human sized aggressive threat at 40 feet. You’re going to need to factor in the second hit probability that you have in your own domicile, but you can do some things to mitigate that risk.
If you need a perfect home defense weapon, this author really likes a shorter barreled, suppressed .300BLK because it gets rid of the hearing problems associated with in-home rifle shooting. It also offers a second shot risk prevention in the slower heavier projectile, and incredible terminal ballistics in that under 50 feet range. Additionally, you can make the gun a very nicely sized carbine that is easy to handle, has generous capacity for multiple threats, and can be accessorized and managed for recoil easily.
A .300BLK with all the “fixin’s” is about the perfect home defense rifle. Sure, there are other cartridges like the 6.8SPC that might look better on paper, but all things considered the .300BLK will do the job, and it will be quiet while doing it. A suppressor is going to require a tax stamp and some hoops to be jumped through.
Nevertheless, all the above listed hunting cartridges are excellent for home defense if you’ve decided on an AR rifle.
I Want to Make Tiny Groups on Paper Targets and Show Them Off
The obvious choice here is the 6.5 Creedmoor. It’s just flat out exceptional at making jagged one-hole targets at ridiculous distances for an AR – but do your best to get a 26-inch barrel if possible (assuming you want to get out past 1200 yards). It’s almost obscene how accurate you can make these guns with the right mix of components, handloaded, tailored cartridges, and a properly made barrel. That said, the 18-20 and even 22” barrels aren’t going to get you out as far as you may have seen the precision long range guys talking about, but the potential with the 6.5 Creedmoor is crazy.
It’s so good it literally challenges the limits of what an AR should be able to do. A .308 Winchester can remain terminal out past 1200 yards. A Creedmoor can go much further and doesn’t have the same drawbacks as the .308 at those distances. It’s wild.
This author can’t outshoot a 6.5 Creedmoor, and on an AR chassis it just makes the most sense for those who want to really improve baseline accuracy potential.
An important note: This is a cartridge that requires the larger AR.308 platform. It cannot fit a standard AR-15. It should not be confused with a 6.5 Grendel. It should also be used with a barrel of longer than 24 inches if possible, and that alone, will add some significant weight to your normal expectation of an AR.
I Need a Gun That Can Do More Than Just One Thing
This may be a more controversial opinion, but given the amazing options listed above, the one this author picks for the “one gun” is probably the least likely pick, and there’s definitely a case to be made. Before the merits of the choice are laid out, it should be noted that any cartridge listed above can do more than one thing for you on the AR platform. They all have great performance in several spaces.
If we had to pick one, it’s the .308 Winchester. Short pause – because of versatility.
Yes, technically it’s not even an AR-15 caliber. It requires the larger .308/AR10 style patterns, and it is going to be bigger, heavier, and more expensive to shoot, but the sheer volume of loads and cartridge options that are available, as well as the finetuning and availability of component parts and accessories makes this the best choice if you had to pick only one gun.
It’s the jack of all trades.
I’d take one all day over a 6.5 Creedmoor, despite the .308 paling in comparison for long range accuracy. For how often one will need to hit on a 1200-yard target, the .308’s 650-750 yard capabilities seem pretty decent. Worst case scenario, I can dial it in with a BDC optic and some basic precision shooting math.
I’d take one all day long over the .223/5.56 because I could drop a sabot 55 grain projectile if I needed to and dial up or back the powder charge to achieve anything I need to. They are also offering 200+ grain projectiles for the cartridge – the options are basically endless.
The .308 components are robust and durable; made to handle quite the pressure load. One could feel confident in long-term durability, easily.
Again, it’s going to cost more, and it’s going to be a bit harder to justify spending more on ammunition, but if the rest of the world fails, this is a great rifle to have if there’s room for only a single one. The zombies will be literally blown away, and the plentiful ammunition from surplus Korean war era cases will still be around for the next 50+ years.
What Are The Basic Numbers When It Comes To The Different Calibers?
Here are the basic numbers for the various cartridges covered in this article and how they compare to the rounds that are meant for each platform to give you a base comparison. We didn’t cover the whole list of available cartridges or even calibers but this article would be 20k words if we tried to do that, and it’s already quite long.
The shorter cartridges that fit natively into the AR15 rifles
Cartridge: .223 Remington
- 55 grain weight; ~3250 velocity; ~1255 ft. lbs. energy
- 77 grain weight; ~2750 velocity; ~1300 ft. lbs. energy
Cartridge: 5.56×45 NATO
- 55 grain weight; ~3255 velocity; ~1290 ft. lbs. energy
- 62 grain weight; ~3150 velocity; ~1360 ft. lbs. energy
- 125 grain weight; ~2205 velocity; ~1360 ft. lbs. energy
- 220 grain weight; ~1000 velocity; ~510 ft. lbs. energy
Cartridge: 6.5 Grendel
- 90 grain weight; ~2875 velocity; ~1650 ft. lbs. energy
- 120 grain weight; ~2700 velocity; ~1940 ft. lbs. energy
- 115 grain weight; ~2575 velocity; ~1690 ft. lbs. energy
- 120 grain weight; ~2450 velocity; ~1610 ft. lbs. energy
Cartridge: .224 Valkyrie
- 60 grain weight; ~3455 velocity; ~1590 ft. lbs. energy
- 70 grain weight; ~3015 velocity; ~1510 ft. lbs. energy
The Longer Larger Cartridge Options (For The AR10/.308 AR Rifles)
Cartridge: .308 Winchester
- 150 grain weight; ~2815 velocity; ~2640 ft. lbs. energy
- 185 grain weight; ~2490 velocity; ~2590 ft. lbs. energy
Cartridge: 7.62x51mm NATO
- 147 grain weight; ~2775 velocity; ~2560 ft. lbs. energy
- 175 grain weight; ~2550 velocity; ~2620 ft. lbs. energy
Cartridge: 6.5 Creedmoor
- 120 grain weight; ~3020 velocity; ~2410 ft. lbs. energy
- 143 grain weight; ~2700 velocity; ~2280 ft. lbs. energy
The Economics Of the Cartridges *(In Relative Terms and Why)
Using a range of 1 through 5 to show relative price comparisons, with 1 being the cheapest and 5 being the most expensive cartridges in this list, here is a basic idea of what you can expect to pay relative to the going market rate for ammunition between “peers” on this list.
Generally, the range for ammunition in times where there is a lot of ammunition on the market is about .45 cents a shot, to $3.25 a shot depending on how premium you want to buy. But there are so many variables that go into pricing of ammunition that a whole article longer than this could talk about the nuances of supply/demand/raw materials, and production factors to explain it and still not be comprehensive enough or easy enough to read. Hence the reasoning for relative price comparisons. Also, the market for ammunition has been “odd” the past couple of years.
.223: Let’s assume the baseline is at .223 and 5.56 and that each of these, though they can vary in price a bit, depending on different loads, is a *2*. They aren’t crazy cheap, but they aren’t particularly expensive either, in the world of the centerfire rifle cartridge.
- 5.56×45: *2*
- .300BLK: *3*
- 6.5 Grendel: *3.5*
- 6.8SPC: *3*
- .224 Valkyrie: *3*
- .308 Winchester: *4*
- 7.62x51mm: *4*
- 6.5 Creedmoor: *5*
Wrapping Up the Caliber Argument For the AR Platform
Because of the difficulty in covering so many rounds in such a confined space, the preference was given to rounds that fit within the normal organic limitations of the AR platform, to include high powered, high velocity (with the exception of the subsonic loads for the .300BLK) Centerfire rifle cartridges.
Other honorable mentions come to mind, including the block busting, brush conquering stubby rounds like .450 Bushmaster, 458 SOCOM and 50 Beowulf. There are others too, that warrant an exploration, like the 300 HAM’R and the .350 Legend and the 7.62×39, but alas those didn’t fit as neatly into this exploration because it starts to get pretty esoteric when you move so far outside of the original constraints of the platform to include such interesting, but ultimately quite niche types of innovation.
There is little question, based both on the fun they provide, and the huge sales numbers that these types of newer cartridges provide, that these are worthy options. But we do feel the mainstream, popular cartridges we highlighted here can offer a very good primer for what you can achieve when you look at the AR platform and try to decide which is the best mainstream caliber or cartridge for use in it for your needs.
Alice Jones Webb is a writer, life-long hunter, experienced shooter, and mother of 4 up-and-coming shooting and outdoor enthusiasts. She grew up flinging arrows and bullets at Virginia whitetails, turkey, and game birds, but her favorite hunting experience is chasing bull elk in the Colorado backcountry. Never one to sit still and look pretty, Alice is also a self-defense instructor and competitive archer. She currently resides in rural North Carolina with her children, non-hunting husband, and a well-stocked chest freezer. You can find more info on me here.