While this debate has raged through the decades, flaring with every significant improvement and/or change in firearms design and ricocheting off of every military contract and police department purchase -- the subject has grown even more complex of late, due to the introduction of dedicated M4/AR-15 cartridges such as the 6.8 SPC, 6.5 Grendel and .300 Blackout.
Yes, I know: Many deserving carbine cartridges such as the 7.62x39mm want in. And sure, some of them will leave size-14 combat boot-prints all over the cartridges just listed. But much as I deplore the necessity, let's confine this discussion to rounds in one arena: those appropriate for the M4/AR-15 family. Otherwise this article would turn into a book.
Trouble is, each popular carbine cartridge has significant strengths, especially in certain areas. What will work best really depends on you and your shooting abilities, your environment and lifestyle, and the probable demands those unique elements will place on your carbine.
Here's a close look at the strengths and weaknesses of a handful of the best AR-15 cartridges available on today's market. Only you can decide which is best, so be sure to vote for your favorite.
While I've never personally drunk the 6.8 SPC Kool-Aid, a lot of very knowledgeable shooters have. It's second only to the 6.5 Grendel — which is a much less available round — in gap-bridging performance. While it doesn't shoot as flat as a 5.56mm or hit as hard as a .300 Blackout, it hits quite a bit harder than the 5.56mm. It also shoots notably flatter than a .300 Blackout, punching a 110-grain Hornady V-Max out at 2,550 fps. Significant body taper enhances reliability under hot and dirty conditions.
On the con side, the 6.8 SPC doesn't feed reliably through standard M16-type magazines, no matter what enthusiasts claim. Some mags work; some don't. You're best off with magazines designed specifically for the 6.8 SPC. And whatever you do, don't call it the '.270 Short ' and boast that it offers performance just shy of the classic .270 Winchester big game cartridge. It's a runt in comparison — don't insult the legend.
Enormously good at some things and marginal at others, the .300 Blackout is particularly capable within 150 yards and pretty much loses it outside that range. Up close, it hits harder than the others on this list. It has the undeniable cool factor of being a proper cartridge for full sound suppression; with 208-grain and heavier bullets traveling less than the speed of sound, a good suppressor makes it quieter than a vulture's sneeze.
Within reason, it's actually pretty versatile too. Load your carbine with 110-grain hunting bullets at about 2,400 fps and you've turned your piece into a deer slayer, as long as you keep ranges reasonably short. Hey, even the renowned .30-30 is considered a 200-yard cartridge at best, and it shoots heavier bullets faster.
The Blackout's weaknesses really come out past 200 yards. If you rarely shoot past there and want a little more authority, this could be your poison.
Although it carries just a little less velocity than similar-weight projectiles fired from NATO's 5.56mm cartridge, and is typically — but not always — somewhat less accurate due to complex bullet design, the 5.45x39mm is very comparable to the 5.56mm for practical purposes out to 200 or 300 yards. That is, with one substantial advantage: 5.45x39mm ammo is currently much cheaper and more available in the current market than either .223 or 5.56mm ammo.
For personal protection purposes, terminal performance of said complex projectile is pretty good; the soft steel core has a flat point, leaving air space inside the tip of the copper-jacketed FMJ bullet, plus a lead plug in its base, making it butt-heavy. On impact, it tends to deform and yaw violently, causing considerable tissue damage.
A few AR-15s are currently manufactured in 5.45x39mm, and ammo is readily available in most large sporting goods stores with a slant for vintage arms.
If you want an AR-15 with classic, fast, centerfire .22-caliber performance but are horrified by the cost of ammo, consider one in 5.45x39mm. Those high-round-count practice sessions will hurt a lot less.
Most times I'm pretty good at hiding my emotions, but this time I won't try: The 6.5 Grendel is my favorite. What other AR-specific cartridge shoots a streamlined projectile that stays supersonic to 1,200 yards? None even come close. It shoots more bullet weight faster than either the 6.8 SPC or .300 Blackout, and is inherently superbly accurate. It's the only cartridge that fits in a standard AR-15-size rifle (less the barrel and bolt), while outperforming both the .223/5.56mm and .308/7.62mm — in terms of less wind drift, flatter extreme-range trajectory and more energy. Its only downside is magazines only hold about 22 rounds of ammunition.
Ammo is not particularly common yet, but if the 6.5 Grendel receives its due, it will be before long. Rifles are becoming available from a variety of manufacturers. If you don't mind stocking uncommon ammo — and handloading, if necessary — and want the most broadly versatile round available filling the pipe of your carbine, the 6.5 Grendel is just magic.
Though the .223/5.56 NATO rapidly gained a reputation as a 'mouse cartridge ' and for anemic stopping power in Vietnam, bullet development in the half-century since has pretty much resolved those issues. When loaded with a premium, purpose-built self-defense projectile, the 5.56mm is plenty potent. While the armed forces still deal with the inadequacies of FMJ projectiles, civilians can load their bedside carbine with soft-point, expanding bullets that leave devastating, bad-guy-stopping wounds.
For practical purposes the .223/5.56mm is a solid 300-yard cartridge. It shoots very flat, recoils politely and imparts a reasonable amount of energy at those distances. It's inherently very accurate, so in a good rifle a skilled shooter can connect at much greater distances, but impact energy is rather low.
One of this cartridge's great appeals is it's very easy to shoot. Its low recoil doesn't intimidate beginners, and making fast follow-up shots is possible. Magazines also hold a tremendous amount of ammunition; more than any of the other cartridges listed here — except for the .300 Blackout.
Is this cartridge for you? Probably. It's common, versatile, easy to shoot, and in normal times, ammo is not hugely expensive. Gun parts and reloading components are common. I'd go so far as to say unless you want an AR-15 for specialized purposes, you'd be well-advised to look no further than the .223/5.56mm.