Along about 1983, Jeff Cooper—the man who founded Gunsite Academy and authored the book, “The Art of the Rifle”—conceptualized the Scout Rifle. Cooper felt a general-purpose rifle needed a general-purpose cartridge, and decided on the .308 Win.; a cartridge capable of pushing a 150-grain bullet to about 2,700 fps, from carbine-length barrels. This level of performance is unobtainable with an AR-15. So, for those looking to employ that platform in the general-purpose role—as envisioned by Cooper—what’s the best cartridge/load for the job?
It should carry enough velocity for bullet upset out to 300 yards. Trajectory should be flat enough that first-round hits on adversaries, or potential meals at that distance, are not problematic. And, big-game and barrier-defeating—general-purpose—ammo, loaded with bullets that expand and penetrate deep, should be commercially available and relatively easy to obtain. You should not have to trade your first-born or spend all night handloading to acquire a full ammo can. Here are some suggestions for the best general-purpose cartridges/loads for the AR-15.
.223 Rem./5.56 NATO This is clearly the most used general-purpose cartridge in the AR-15, but projectiles are light, so monolithic or bonded bullets that hold together and drive deep are called for. At the top of the list is Remington’s HTP Copper load, topped with the 62-grain Barnes TSX bullet. The Nosler Defense 64-grain Bonded and Speer 64-grain Gold Dot loads are right with it. All will perform well against intermediate barriers and provide good terminal performance.
224 Valkyrie The Valkyrie is all about extending the range of the AR-15. Its highly modified 6.8 SPC case holds more powder, and the fast-twist barrel handles extremely aerodynamic bullets. Currently load options are limited, but the 90-grain Federal Premium Fusion will perform similarly to a monolithic bullet by driving deep and creating wide wound cavities.
25-45 Sharps The premise with the Sharps was to deliver .250 Savage performance from an AR-15. From carbine-length barrels it will push an 87-grain bullet beyond 2,800 fps, and unlike most .22-caliber centerfire cartridges, it is big-game legal nationwide. The 87-grain Speer Sharps Rifle Company load is the best option, but the cup-and-core bullet lacks the integrity for ideal tactical application.
6.5 Grendel With bullets heavier than 100 grains, the Grendel can deliver the flattest trajectories from an AR-15. Like with the Valkyrie, the Federal Premium Fusion load is probably the best general-purpose option. It has the toughness to hold together, drive deep and maximize wounding, even when hitting heavy bone or intermediate barriers.
6.8 SPC Designed with the hopes of pushing the military into a new standard battle rifle chambering, for a time the 6.8 SPC was thought to be the next best thing in the AR-15. The cartridge lacks the velocity to guarantee ideal monolithic bullet upset at distance, so the best general-purpose option is, very likely and once again, the Fusion MSR offering from Federal Premium.
.300 BLK Fantastic as a subsonic cartridge, the .300 BLK is somewhat anemic in its supersonic form. Out of carbine-length barrels it struggles to surpass Mach II, even with its lightest projectiles. Though not a true ballistically capable general-purpose round, if the .300 BLK is what you’re using, consider Hornady’s 135-grain FTX bullet. The polymer tip will help the projectile expand and deliver maximum tissue destruction at low-impact velocities.
.300 HAM’R By duplicating the ballistics of one of the greatest hunting cartridges of all time—the .30-30 Win.—the new .300 HAM’R is the latest counter to those who claim that the AR-15 is not suitable for hunting. Though ammunition selections are limited, the cartridge has much to offer. Wilson Combat’s 110-grain Controlled Chaos load is likely the best general-purpose option. Thanks to its unique design, it is more than capable of delivering barrier-defeating lethality up close and at distance.
7.62x39 mm Due to extreme case taper, this cartridge has a poor reputation for reliability in AR-15s, but affordable ammo abounds for rifles that cycle it dependably. For general utility, the cartridge’s low downrange velocity is a hindrance. Federal’s 123-grain Fusion load should deliver the best balance of penetration, expansion and trajectory in this chambering.
Of course, none of these cartridge/load combinations come close to the .308 Win. Cooper selected as being ideal for general-purpose use. Still, they all recoil less and some shoot just as flat. In an AR-15 they’re much lighter than any semi-automatic or commercial Scout Rifle chambered for the 7.62 NATO. Similarly, 100 rounds of .308 Win. weighs twice as much as 100 rounds of .223 Rem. When on foot and in peril, that matters.
Specialized loads for the range, hunting, or for certain tactical applications, are a good thing. But, here in the United States the AR reigns as the supreme utilitarian rifle. Having a coast-to-coast and a border-to-border cartridge/load combination that’ll do everything you need a rifle to do, and do it well, is the pragmatic approach. For many reasons, the AR-15 can never be a Scout Rifle, at least as defined by Jeff Cooper. But, with the right cartridge/ammo combination it can for sure serve the role of the general-purpose rifle.