The AR Platform: Best Calibers by Purpose

One of the many benefits of the AR platform is its modularity. By swapping out the complete upper receiver assembly and magazine, you can convert your standard 5.56mm NATO AR to .300 Blackout, 7.62×39, 6.8 SPC, 6.5 Grendel, .224 Valkyrie, and a variety of other calibers without having to purchase a dedicated rifle for them (Wikipedia has a reasonably complete list HERE). In fact, the design of the AR is so successful that other variations have been made with lower receivers for other calibers that don’t fit that standard AR-15 lower specifications.

With all of these options available, some people may find themselves asking “what is the best caliber for my AR?”, not realizing that they aren’t asking the right question. There is no “one caliber to rule them all”… calibers that are effective in close-quarters engagements typically can’t function at long range; likewise, calibers designed for long distance engagements are often too powerful for close range. Consequently, the real question one should ask is: “What caliber is best for my use case?” And to answer that question, you need to know what you’re going to be using your AR for.

Home Defense & Personal Protection

In home defense and personal protection situations, the maximum engagement distance will rarely exceed 25 yards. Most engagements will occur inside or immediately outside of a building, structure, or vehicle, and over-penetration is a significant concern. This application calls for a compact and maneuverable firearm, often referred to as a Personal Defensive Weapon (PDW); it should have a short barrel so that the user can more easily manipulate it in tight quarters, and be chambered in a caliber that will present the lowest risk of over-penetration without compromising effectiveness in terminal ballistics.

Based on the use case and assumptions we’ve just described, an AR pistol would fit this niche nicely. A rifle would require a 16″ barrel at minimum, which would not be as easy to use when maneuvering in confined spaces. While a traditional pistol would be adequate at that distance, the AR platform would allow for an extremely short barrel, and the use of a stabilizing brace for enhanced accuracy and control.

Unfortunately, most calibers the AR is chambered for are intended to run in much longer barrels. For example, the 5.56mm NATO cartridge was intended for use in a 20″ barrel; the effectiveness of the round diminishes substantially (and barrel pressure increases significantly) for every inch shorter your barrel gets. The .300 Blackout is also proven effective in barrels as short as 6″, but comes with a significant risk of over-penetration; in an urban environment, this presents a very real concern for the safety of innocent bystanders.

This is one case where a pistol caliber would actually be preferable to a rifle caliber. Sure, a 9mm or .45 ACP can punch through drywall just as easily as .300 Blackout, but the energy of either caliber would dissipate much faster. They are also known to function reliably in all lengths of barrel, and defensive ammunition designed specifically to reduce the risk of over-penetration is readily available. Ammunition is comparatively less expensive, weighs less, and takes up less space than rifle ammunition, and with the right setup both ammunition and magazines can be shared with an accompanying sidearm. It is important to note that some defensive ammunition doesn’t cycle well in certain AR pistols – you will need to do some trial and error to determine what defensive ammo your weapon prefers.

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Ideally, if you’re considering a pistol-caliber AR for home defense, you should get one dedicated to that caliber. There are a few ways to approach this:

  1. If you have the money, the best option is to purchase a complete AR pistol made by a reputable manufacturer. Going this route gives you the maximum likelihood of reliability, and a manufacturer’s warranty to fall back on if the product fails to function as advertised. The CMMG Banshee 300 or PSA Gen4 GX PDW are two examples of weapons that would be useful to fill this niche.
  2. The next best option is to buy a dedicated lower receiver in the caliber you prefer, and then a complete upper receiver that is known to function reliably with that lower. Building one in this way can help you save some cash while also reducing the amount of unnecessary spare parts lying around on your workbench.
  3. Buying parts from a variety of manufacturers and assembling your weapon “as cheaply as possible” is technically doable, but it comes with some risk. Manufacturing specs and tolerances are not as standardized for pistol-caliber ARs as they are for AR-15s, so if you mix and match brands when building it you could easily find yourself with a weapon that doesn’t function reliably. In order to succeed with this kind of effort you’ll need to do a ton of research to make sure the parts you’re buying are proven to be compatible.

One thing to bear in mind about dedicated pistol-caliber lowers is that the market is heavily biased in favor of Glock pistol magazines. Finding a lower that takes SigSauer, XD, XDm, M&P, Beretta, or other high-capacity magazines can be a real challenge, and those you do find are probably going to be more expensive than their counterparts that accept Glock mags.

If you just can’t find a dedicated lower that works with your preferred pistol magazines (or if you want the option of converting your AR pistol back to a rifle caliber at some point in the future), you’ll have to stick to a traditional AR-15 lower receiver and use a conversion kit to make it work with pistol ammunition. The main downside to this method is finding a reliable conversion kit, and (potentially) having to learn a new battery of arms when operating it. There are two conversion types you might consider:

  • Magazine well inserts are simply adapters fitted to the AR-15 magazine well that change its shape to allow the use of non-AR magazines. These have been around for years in various forms. The most reliable is probably the Stern Defense kit, but it also happens to be the most expensive option. At over $600 for a magazine well insert and pistol-caliber upper receiver, you would be spending as much as you would buying a new AR pistol in the caliber you want. However, with the Stern version you can convert your AR pistol from 9mm back to 5.56 in minutes.
  • Alternatively, the newest animal on the conversion market at the time of this writing is the EndoMag, which lets you convert a Gen2 or Gen3 PMAG so that it can load 9mm ammunition and be used with a dedicated 9mm upper. As this product is very new, its reliability has yet to be fully proven, and doesn’t allow you to share magazines with your pistol. You will also want to mark your pistol-caliber magazines so that you don’t confuse them with your 5.56mm gear.

Short-Range Combat

The Short-Range Combat use case assumes the need to engage targets at distances out to 300 yards. A hard-hitting rifle caliber is needed to deliver maximum kinetic energy to the targets farther away while still remaining maneuverable enough to operate in close quarters if needed. In a nutshell, this use case calls for a middle ground between the full-sized AR-15 and the ultra-compact PDW mentioned earlier.

Once again, the AR pistol becomes the ideal choice for this scenario. A relatively short barrel is still needed to address the potential for close-quarters engagement, and a stabilizing brace helps ensure a solid platform for accurate shots at distant targets. The ideal barrel length would be between 8″ and 12″, with bias towards the shorter end if possible. An alternative to the AR pistol would be a short-barreled rifle; while these are legal to own in some states after a lengthy background check, waiting period, and $200 tax stamp to the ATF, the AR pistol doesn’t require you to jump through the same legal hoops and delivers many of the same benefits.

From a performance perspective, the ideal caliber for this use case is .300 Blackout. It is designed specifically for use with short barrels, and can accurately deliver heavier bullets with greater energy than 5.56mm. It has similar overall ballistic performance to 7.62×39, but doesn’t require a dedicated lower receiver. The fact that it uses standard AR magazines is huge plus if you have multiple ARs of different calibers.

.300 blackout is also an ideal caliber to run suppressed – with subsonic ammunition it is incredibly quiet, though this drops its effective range to 50 yards. Fortunately, you can always drop in a magazine of regular supersonic ammunition and engage at distance even with a suppressor – it won’t be as quiet, but it will definitely still work as intended.

One key downside to .300 Blackout is the cost of ammunition, which is roughly twice that of 5.56mm. If budget is a consideration for you (and it is for almost everybody), this could easily turn you away from the caliber. Another issue to consider is over-penetration – while not as prominent a consideration in this use case, you have to keep that risk in mind if the potential exists for it to be used in densely populated areas.

It is important to note that 5.56mm does still work for this use case. It’s not the best round for it, but it will get the job done in most respects. If you are adamant about keeping a limited number of calibers on hand, you can get by in this use case with a 10- or 11-inch upper chambered in 5.56mm NATO. If you choose to go this route, you will want to invest in some quality ammunition designed for use in shorter barrels. Remember, you can always try out a dedicated .300 Blackout upper – if you don’t like it, all you have to do is get a different upper in 5.56.



General Purpose Weapon

The general purpose weapon is the Swiss Army knife of firearms – it can be used in virtually any situation from close range out to 500 yards. With the need to engage targets effectively at distance, the barrel length will necessarily be greater than the two previous use cases.

A standard AR-15 with a 16″ barrel (or 14.5″ barrel with flash hider) was designed for this exact use case. The barrel length makes it ideal for reaching targets at greater distances, but it isn’t so long that it becomes a serious impediment in close quarters situations.

For this use case, the 5.56mm NATO caliber is perfect for a number of reasons. For one thing, it’s a great utility caliber. While it may not be the best choice for very specific applications, its generally good ballistic performance at a variety of engagement distances makes it ideal for a general purpose weapon.

Another benefit of 5.56mm is cost – it really isn’t that expensive to purchase, especially in bulk. Cheap ammo for practice sessions is easy to come by, as are a wide variety of specialty rounds for different applications. Tailoring your load of ammunition will help greatly with ballistic performance in each use case. For example, consider getting hollow-points for close range engagements – they will expand on impact, reducing the energy of the round quickly and lessening the risk of overpenetration.

You will likely want to equip the rifle with some sort of magnified optic in order to better engage those distant targets. A red dot with a magnifier that flips to the side will allow the user to meet both short-range and mid-range needs. Conversely, you could also use a 3x scope for those targets at 200-500 yards, and mount a pair of offset sights for close-range encounters.

Long-Range Marksmanship

This use case calls for precision accuracy past 500 yards, with maximum kinetic energy at point of impact. This weapon will need to be equipped with a reasonably powerful scope, and have a longer barrel to increase the velocity and effective range of the bullet.

Depending on the user’s intended engagement ranges, the weapon might be configured in different ways. A designated marksman rifle (DMR) is intended for engagements out to 650 yards, and is typically equipped with a 1-6x variable optic. Comparatively, rifles used for engagements out to 1,000 yards are typically equipped with a scope capable of 10x or better magnification.

With the right barrel length and ammunition, a 5.56mm rifle can certainly handle the role of a DMR. It will not deliver as much energy as other calibers, but ammunition is plentiful and relatively inexpensive, recoil is very light, and it’s the caliber the AR-15 was designed to shoot. However, it is not capable of effectively engaging targets 1,000 yards away.

.308 Winchester (or its near-twin, the 7.62x51mm NATO cartridge) is the most common caliber in marksman, sniper, and hunting rifles, and has been for over 50 years. The .308 can reliably prosecute targets out to 800 yards in all but the most extreme weather conditions, and 1,000 yards is doable with slightly less reliability. In either case, this hard-hitting round delivers good ballistic performance at those ranges, so it can support either DMR or sniper roles. Unfortunately, it does not fit in a standard AR-15 lower receiver – you will need to get an AR-10 lower in order to use .308. Additionally, ammunition is also more expensive, heavier, and produces more recoil in comparison to 5.56.

In either DMR or sniper applications, 6.5 Grendel is a definite winner. This hot little round delivers significantly more energy than 5.56, has less recoil than .308, can reach out to 1,000 yards reliably with decent terminal ballistics, and uses a standard AR-15 lower receiver. While dedicated 6.5 Grendel magazines are preferred, 6.8 SPC magazines will also work with it if needed.

The one downside of 6.5 Grendel is the cost and availability of ammunition – it’s definitely not cheap to shoot. The lowest pricing of $0.29 per round can be found on steel cased ammunition, but if you insist on running brass through your rifles, you’ll be hard-pressed to find it cheaper than $0.80 per round. You really shouldn’t be using a precision rifle to dump mags like you might with an AR, so the cost shouldn’t hit you too badly, but you might have difficulty getting your hands on some in a major crisis. For situations like that, it’s not a bad idea to have some cheap steel-cased ammo on hand so that your rifle doesn’t become a fancy paperweight when you need it most.

If using the 6.5 Grendel for a DMR role, an upper with a 20″ barrel will give you all the performance you need at distances well beyond 700 yards. For engagements out to 1,000, a 24″ barrel will sacrifice maneuverability due to the increased length, but it will give the round even more velocity and effective range. For extreme long-range rifles, you should consider investing in a high quality scope, a comfortable rifle stock, and a durable bipod.

Wrapping it Up

The caliber you select for your AR should be determined by what you intend to use it for.

  • If your main objective is home defense or personal protection, think about getting an AR pistol that accepts the same ammunition and magazines as your preferred sidearm; this may require a dedicated lower, but gives you the most utility for the use case.
  • If you’re mostly needing close-range performance with more punch than a pistol caliber, consider the .300 Blackout first, with 5.56 as an alternate if cost is a big issue.
  • If you want a jack-of-all-trades rifle, the standard AR-15 in 5.56 will get the job done.
  • If you’re looking for either a DMR or a long-distance precision rifle, 6.5 Grendel is hard to beat. 5.56 will work as an alternative for your DMR; .308 would do for longer range engagements, but requires a dedicated lower.

One Last Tip

If there’s anyone that knows the AR-15 platform, it’s the US military. As a special offer for our readers, you can get the Official US Army Manual for AR-15/M4/M16 right now – for free. Click here to snag a copy.

One Last Tip

If there’s anyone that knows the AR-15 platform, it’s the US military. As a special offer for our readers, you can get the Official US Army Manual for AR-15/M4/M16 right now – for free. Click here to snag a copy.

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6 thoughts on “The Best AR15 Calibers

  1. Nice read! You didn’t mention my favorite GP cartridge, the 6.8 SPC, but I am biased. I’m a former SF operator and know Steve Holland who designed it. But that being said, if you covered every cartridge available for use in the AR platform you’d have a pretty heavy book. I have ARs in several calibers, mainly because it gives me one more excuse to get something new. It’s kind of an obsession and quite addictive.

  2. Hi yes very helpful to me as a 1st time ar 15 owner plan on getting into competitions

  3. You know what would be able to shoulder many of the above cartridges aside if only it were available? The 7.62×45. Yes x45. If you never heard of that cartridge it is because it was developed by the Czech military and saw use for only about a decade before being forced out for mother russia approved 7.62×39.
    I came to know it when I acquired a VZ-52 surplus rifle and was more than impressed by this rarely seen cartridge.

    7.62×45, Czech it out. If only we had a decent supply of commercial ammo in this round I think it would be a serious contender in AR platform guns.

    1. You are describing the 300 HAM’R by Wilson Combat.

  4. I use 7.62×39 in both a 10 1/2 and a 16 inch barrel. The ballistics are very similar to .300 blackout for a fraction of the cost. Accuracy with modern decent barrels is very acceptable to 300 yards and enough knockdown power to take white tail, hogs or most medium size game. They are low recoil and total blast for target shooting. There is always an abundance of ammo and there are high quality hunting rounds to.

    1. Exactly what I was thinking. I built 2 in 7.62×39. 16 inch carbine with red dot for short range use and a 20 inch with rifle length gas system for longer range use. That gun has a 3×9 scope on it for now. Both are fun to shoot, good accuracy, and ammo is cheap and everywhere. It works for me and did not require any special lower receiver as some have stated. Hunting at woods ranges is no problem with this round. Success is more likely with a gun you shoot often and the cost and variety of 7.62×39 ammo is hard to beat.

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