Over the course of nearly 60 years the AR-15 has been with us, some 65 recognized cartridges have been designed specifically for it. Most are wildcats based on the original .223/5.56 case; some became relatively popular as such and some went on to achieve SAAMI recognition and loaded commercially by at least one or more ammo manufacturers.
Others sought to base their creations on fatter cases like Remington’s 6.8 SPC, .220 Russian and .284 Win., all of which can hold more powder than the 223/5.56.
The latest cartridge designed around the AR-15, though not limited to it, is Hornady’s 6mm ARC (Advanced Rifle Cartridge), which is already on dealer shelves as you read this. The 6mm ARC was developed by Hornady for the Dept. of Defense, who wanted a cartridge that had better long-range capabilities than the 5.56, yet still fit the AR-15 platform. The ARC certainly does that.
Based on the 6.5 Grendel case, the ARC has a .441” rim diameter, the same as the .220 Russian and the 7.62×39. Other than the neck diameter, the differences between the ARC and Grendel cases are so slight you’d be hard pressed to see it even in a side-by-side comparison.
Both share the same 30-degree shoulder angle, but the Grendel’s body length i.e., the base to shoulder/neck juncture, is .030” longer. Neck length is also in the Grendel’s favor, but only by .010”. For all intents and purposes then, the ARC is essentially a necked-down Grendel with minor dimensional changes.
Unless you already own a Grendel, a .223/5.56 bolt head will have to be swapped out, but that’s no big thing. As for the magazine, you’d think that a Grendel mag would work just fine, but initial reports don’t bear that out. To ensure absolute reliability, a caliber-specific magazine is probably called for. And speaking of reliability, because the ARC case has nearly double the rim thickness as the .223/5.56, it makes for more reliable functioning, especially in full-auto military arms.
Okay, so what is Hornady claiming for their newest baby? Well, in terms of absolute 6mm performance, it’s a relative pipsqueak. But remember: it had to fit the AR15/M-16 platform and this case is about as fat a cartridge as can be digested.
And overall cartridge length is limited, as well, to 2.26”. In other words, the .243 Win. is a magnum compared to the ARC. Nevertheless, the 6mm ARC definitely has some things going for it, most of which have to do with the incredibly efficient bullets we have available today.
Back in the day, your typical .243 Win.100-grain bullets had a G1 BC (Ballistic Coefficient) of around .360-.375 and were launched in barrels with a 1-10” twist. The three loads Hornady has developed for the ARC is a 103-grain Precision Hunter bullet with a BC of .512; a 105-grain BTHP with a .530 BC and a 108-grain ELD-Match with a BC of .536.
Those BCs in a 6mm bullet were unheard of years ago. To stabilize these heavier, longer bullets, a twist rate of 1-8” is required. Through 24” test barrels, Hornady lists MVs of 2,800, 2,750 and 2,750 fps, respectively.
Most ARs destined for hunting will have barrels shorter than that, so velocities will suffer commensurately. But, if launched at nominal velocity through a 24” barrel, Hornady’s 108-grain ELD Match load will still be stabilized at 1,250 yards. Who would have thought that a modest capacity 6mm could be competitive in 1,000-plus yard competition?
I can remember when the .243 Win. — and to a much lesser extent, the .244/6mm Rem. — were all the rage from the time they were introduced in 1955 to the mid-`60s as “dual purpose” cartridges, but the .24 caliber itself has long since fallen from favor with hunters.
As to how this less able cartridge will catch on — the fact that it’s designed for the AR-15 platform notwithstanding — remains to be seen. As of this writing more than 24 firearm and barrel manufacturers are listed on Hornady’s website as chambering for the 6mm ARC, and the list will only grow from there. Time will tell.–Jon R. Sundra