Top 5 Most Useful AR-15 Cartridges

In addition to being arguably the most influential rifle design of its 52 years, the AR-15 is also the most controversial. According to many shooters, its alleged weaknesses stem from the cartridge it chambers. For the better part of its life, the M16/AR-15 digested only one kind of ammunition: the twin cartridges labeled 5.56 NATO and .223 Remington.

Currently, the 5.56/.223 is still the most practical choice for the AR-15. However, a myriad of other AR-15-specific cartridges of more-or-less usable design have cropped up in the last decade or two, including the .300 AAC Blackout (SAAMI-approved in 2011); the 6.8 Remington SPC (2004); the 6.5 Grendel (2003); .25-45 Sharps (2014); the .458 SOCOM (2001); .50 Beowulf (2003); and the spanking-new .22 Nosler (2017).

Additionally, wildcat cartridges for the AR-15, such as the 6mm/.223, abound. However, they're not readily available, so for the purpose of this article, we'll stick to SAAMI-approved factory rounds.

What's the motivation behind all this cartridge innovation?

Most frequently, new-cartridge designers point out real and perceived weaknesses in current cartridges (usually the 5.56/.223) and make sometimes practical, sometimes outrageous claims as to how their new cartridge overcomes those weaknesses. My personal favorite was Remington's statement that its new .30 Remington AR cartridge (introduced in 2008) was the ballistic equal to the .308 Winchester — and to prove it, the company provided ballistic charts contrasting the new short, fat AR cartridge with its 125-grain Reduced Recoil .308 load. Clearly, the latter does not showcase the .308 at its best; shooters saw through the farce, and the .30 Remington AR — quite a capable cartridge on its own terms — failed.

While most of the "new and improved" AR-15 cartridges offer certain advantages over the original M16/AR-15 round, all but one are less versatile. For example, the .300 AAC Blackout (which is simply a slightly modified version of J.D. Jones's .300 Whisper wildcat) provides distinctly more close-range authority than the 5.56/.223 and is particularly useful in suppressed firearms, but it falls apart at long range.

Be that as it may, AR-15 enthusiasts have choices, and that's a beautiful thing.

Here's a look at what I consider to be the five most useful AR-15 cartridges available today, along with a brief shopping list of necessary conversion parts for each.

In full disclosure, I've left out the big-bore thumpers such as the fascinating .50 Beowulf and .458 SOCOM. Simply put, they're so specialized that they're a little beyond practical.

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Also — and I can hear the wails of disbelief — you'll note that the 6.8 SPC cartridge isn't included. Why? It offers performance so indifferent in every category that it's uninteresting to me. It's not good at long range because it can't shoot high-BC bullets, it's not good for hunting because it can't shoot bullets with adequate sectional density fast enough, it's not good for competitive use because accuracy isn't particularly special and recoil is a bit high for fast work, and it's not as authoritative at moderate distances as several of the others detailed here. Worse, unless one is able to obtain ammunition loaded to the zesty pressure levels originally advertised by designers, performance is less than ho-hum.

I can hear your gnashing teeth already. If I left out your favorite, well, cowboy up and try one of these real AR-15 cartridges. And yes, the 5.56/.223 original M16 cartridge is one of them.

May as well start at the top, right? Debatably, the 6.5 Grendel is the only cartridge that can challenge the 5.56 for the title of AR-15 versatility champ. And if it weren't for the vast variety of projectile choices available for the 5.56, it wouldn't even be in the same arena as the 6.5 Grendel.

Designed by Bill Alexander and cohorts, the 6.5 Grendel uses the .220 Russian as its parent case. If that raised your eyebrows, screw 'em back down before anybody notices — the .220 Russian is also the parent case for the world-championship-winning PPC family of benchrest cartridges.

According to Hornady's 9th Edition of Cartridge Reloading, the 6.5 Grendel "...is quite possibly the most efficient cartridge ever to be chambered in the AR platform." Quite a resounding accolade from a company that created the most popular 6.5mm cartridge in history (the 6.5 Creedmoor).

My personal 6.5 Grendel is an 18-inch Hunter model by Alexander Arms with a side charging handle, and it pushes a 123-grain Hornady A-Max at a rather pedestrian-sounding 2,440 feet per second (fps). However, the projectile is slippery enough (its BC is .510) that it stays supersonic to 1,000 yards, even in the dense atmospheric conditions encountered at sea level, giving it bona fide long-range capability. Push that same projectile to 2,580 fps out of a 22- or 24-inch barrel, and it's even better.

For big game such as deer, antelope and all but gargantuan black bears, handload a 120-grain Nosler Partition or Swift A-Frame to the same velocity. Inside of 300 yards — or even 400 in the hands of a very good rifleman — the 6.5 Grendel is a high-capacity venison hammer that offers follow-up shots faster than any bolt action.

Predators? Its only weakness on varmints of the canine persuasion is the lack of very high muzzle velocity, which means that the 6.5 Grendel doesn't have the laser-flat short-term trajectory (inside 300 yards) of a .224 bullet going 3,200 fps. However, a 95-grain Hornady V-Max or Nosler Ballistic Tip pushed to 2,700 fps absolutely flattens coyotes and foxes; you'll just need to know your trajectory and perhaps carry a rangefinder.

On the subject of high velocity, this new hot-rod .224 cartridge pushes varmint bullets very fast indeed — up to 3,500 fps with 55-grain versions. Nope, that's not quite as fast as the .204 Ruger with 32-grain bullets or the .17 Remington with 20-grain bullets, but neither does it have the will-o-the-wisp characteristics in the wind that plague those cartridges.

Better yet, the .22 Nosler pushes a 77-grain Sierra Tipped MatchKing bullet at 3,100 fps, and with its relatively fast 1:8-inch twist rifling, it stabilizes them beautifully. My handload averages .47-inch three-shot groups at 100 yards. While specialty 5.56 loads achieve honest 1,000-yard capability, the .22 Nosler makes shooting 1,000 yards with an AR-15 almost a cakewalk.

In local atmospheric conditions near my home (5,050 feet altitude, 50 degrees Fahrenheit and so forth), that speedy 77-grain TMK stays supersonic to exactly 1,300 yards. This is an amazing feat for an AR-15 cartridge.

To the surprise of many shooters, engineers chose to employ an entirely new cartridge case when developing the .22 Nosler. Use of a rebated rim enabled designers to utilize standard 5.56 bolts, and a fairly steep 30-degree shoulder maximizes capacity without compromising reliability.

Two loads are initially available from Nosler: a 55-grain varmint and predator load and a 77-grain Custom Competition load. I suspect that eventually we'll see a Partition or Bonded Solid Base bullet for deer-size game.

As for magazines, here's where the woeful 6.8 SPC (tongue in cheek, folks!) actually contributed to the future of great AR-15 cartridges: Magazines manufactured for the 6.8 SPC are perfect for the .22 Nosler.

Without a doubt the second-most-popular AR-15 cartridge currently available, the .300 AAC Blackout also possesses the distinction of being remarkably focused. As in, not versatile. However, the unique advantages of the cartridge make it both critically important within the AR-15 realm and eye-bulgingly fun for those who give it its due.

While its first appearance in wildcat form is rightly attributed to J.D. Jones, the .300 BLK (official SAAMI short moniker) was refined for the AR-15 around 2010 and was intended to provide performance characteristics similar to that of Russia's legendary 7.62x39 fighting cartridge. According to SIG Sauer's John Hollister, the .300 AAC Blackout was optimized for maximum efficiency in a 9-inch barrel, making it admirably well-suited for clandestine work in compact AR-15 SBRs.

The parent case is officially the .221 Fireball, and its ancestry harks back to the .222 "Triple Deuce," predating the 5.56/.223. However, in common terms, the case can be thought of as a modified .223 case necked up to accept .308-diameter projectiles and trimmed to a much shorter 1.378 inches.

As a result, the .300 BLK functions perfectly with standard AR-15 bolts, magazines and everything else. All you need is a new barrel.

Most factory loads for the .300 BLK feature either a very light or a very heavy 30-caliber projectile. Heavy versions are loaded to subsonic speeds, which don't disturb the sound barrier and create the distinctly audible sonic crack; in other words, when paired with a quality suppressor, the cartridge can be very quiet indeed. Typically, subsonic .300 BLK loads push match bullets of 200 grains or more, and while they're the essence of AR-15 subtlety, they have limited use at anything beyond close range.

The lighter loads utilize 90- to 125-grain bullets and achieve around 2,200 to 2,600 fps. Obviously, suppressing these isn't nearly as effective as suppressing subsonic projectiles, but they're still not obscenely loud since they don't burn much propellant. Generated muzzle energy ranks from 1,300 to 1,500 foot-pounds, just a few percent short of that provided by the 7.62x39. Plus, although the aerodynamics of the short, fat projectiles is abysmal, inside of 200 yards or so they provide very usable fighting-grade ballistics.

Admittedly, including this cartridge was a bit of a gamble. It's not particularly well-known, although it deserves to be, and it has not experienced the growth necessary to bring its full potential to fruition. That said, it's a superb little hunting cartridge that matches the then-spectacular performance of the classic .250-3000 Savage cartridge. Introduced in 1915, the .250-3000 was the first cartridge to achieve 3,000 fps muzzle velocity, and at the time it was the deer cartridge to have.

In design, the .25-45 Sharps is simply a .223 cartridge case necked up to 25 caliber. In fact, it's basically a simplified and legitimized version of the .25-223 wildcat, which is the same thing with the case shortened .050 of an inch to allow the use of longer-ogive 25-caliber projectiles. According to designer Jay Lesser, he and his team at Sharps Rifle Company engineered it that way to make forming .25-45 Sharps cases from .223 brass as easy as possible. Plus, shooters with a supply of .25-223 brass can safely fire it in their .25-45 Sharps chamber.

Like the .250 Savage, the .25-45 Sharps will push an 87-grain soft-point bullet to 3,000 fps out of a 24-inch barrel, giving it legitimate authority on deer-sized game. Lighter bullets in the 70-grain range can be pushed to 3,200 fps, offering serious predator-thumping ability. My favorite handload features an 80-grain Barnes TTSX bullet at 2,925 fps; the tough projectile penetrates better than factory-loaded bullets and groups less than ¾ MOA.

Because it is best suited with light-for-caliber .257-diameter bullets, and .257 bullets don't have the greatest aerodynamics to begin with, the .25-45 Sharps isn't great at long range. But it's good for deer and smaller game, it's easy and economic to reload, and it offers about 10 percent more muzzle energy than the 7.62x39mm — which is famous as a manstopper.

Ah … the all-around champion. Because the armed forces have been stuck with the 5.56 NATO cartridge for over a half-century, special R&D teams inside and outside the military have made a point of developing performance-enhancing loads. As a result, the cartridge has become far more capable under a vast array of demands than anyone fighting in the jungles of Vietnam could have imagined.

Nontoxic projectiles? No problem. Penetrators? Sure. Low-flash self-defense ammo? Absolutely. Long-range, 1,000-yard-capable ammo? Right this way. From speedy featherweight projectiles screaming out of the muzzle in excess of 4,000 fps to heavy, so-long-you-have-to-single-load bullets designed for maximum range, ammo for the 5.56/.223 runs the whole gamut.

Purely because of the almost limitless versatility of available ammunition, the 5.56/.223 is still the practicality champ when it comes to AR-15 cartridges.

Because I don't have nearly enough room to discuss all the various genres of 5.56/.223 ammo, let's just take a brief look at a few of my favorites.

On the lighter side, Hornady's 40-grain V-Max load usually shoots obscenely tiny groups even in the faster 1:8-twist ARs and achieves somewhere close to 3,800 fps. As a result, it's pure poison on predators.

Jumping up to a medium-weight projectile, I've had tremendous luck with Hornady's 60-grain V-Max in terms of accuracy. Almost every AR-15 I've fired it through shoots it into sub-1-inch groups, even those that aren't built for accuracy. Plus, it fragments violently on impact, making it ideal for personal protection in densely populated urban environments where you really want bullets to stop in a violent threat's body or — heaven forbid you miss — fragment in the wall instead of going through and endangering your neighbors.

All that said, though, my two all-time favorite loads for all-around work from zero to 1,000 yards are Barnes' 85-grain Precision Match, which is a superbly accurate load that I've won many a competition with, and Black Hills' 77-grain Tipped MatchKing .556 load. Both perform admirably to 1,000 yards, expand dramatically in ballistic gelatin and shoot with eyebrow-raising consistency. Amazingly, the Barnes load is supersonic to 1,090 yards, and the Black Hills load to 1,120 yards in my local atmospherics, giving both versions real-world, long-range effectiveness.

While they're not as fast and flat inside of 300 yards, making it slightly more difficult to connect with a coyote, I know there's no challenge inside of 1,000 yards that I can't at least make a credible attempt at meeting. And that, my friends, is versatility.