6MM ARC
Rifle|Fall 2021
After using a 6.5 Grendel to cull a goodly number of Texas feral hogs, I’ve developed a great deal of respect for the cartridge. This has mostly involved nighttime forays shooting with thermal imaging optics. The 2.26-inch confines inherent to AR-15 magazines, and the Grendel’s limited case capacity, make 123- to 130-grain bullets the practical upper limit for such activities. These projectiles chug along at around 2,350/2,450 feet per second (fps), but deliver well out of proportion to its diminutive size.
PATRICK MEITIN

Now, take the 6.5 Grendel case, neck it to 6mm, give firearms fast rifling to handle long-for-caliber bullets weighing from 103 to 115 grains, including ballistic coefficients (BC) equaling those 6.5 projectiles, and you have Hornady’s spanking-new 6mm Advanced Rifle Cartridge, or ARC.

As the story goes, Hornady developed the 6mm ARC under direction from the U.S. Department of Defense, optimizing the Grendel case to meet prescribed goals. The case head remains .441 inch and the rim is .059-inch thick. The neck was set back, allowing more room for long-for-caliber 6mm projectiles in standard AR magazines. The shoulder angle was set at 30 degrees. The cartridge was quickly accepted as a Sporting Arms and Ammunition Manufacturers’ Institute (SAAMI) standard, an excellent start to such a new cartridge. It could easily be argued the ARC is yet another version of the 6mm PPC (which was never SAMMI standardized) – actually a pretty fair assessment, though the fast rifling twist changes everything.

Hornady launched the 6mm ARC with three factory loads: a 103-grain ELD-X Precision Hunter at an advertised 2,800 fps, 105-grain BTHP Hornady Black at an advertised 2,750 fps and the 108-grain ELD-Match load at a stated 2,750 fps. Velocities were established from a 24-inch barrel.

In very simple terms, the 6mm ARC’s exterior ballistics exceed anything currently possible with any other AR-15 cartridge, especially at long ranges. It blows the doors off the 5.56 NATO/.223 Remington, providing reduced wind drift and higher retained velocity/ energy at any range. It even beats the 6.5 Grendel at extended yardages. The 6mm ARC also betters or equals AR-10-based .308 Winchester ballistics regarding drop and wind drift (if not delivered energy). Loading the longest-for-caliber .308 bullets provides equal wind drift and velocity retention, but at the cost of heavier recoil.

The 6mm ARC as a possible military round doesn’t interest me so much as the promise of a serious hunting round. Flat trajectory minimized wind drift, high sectional density to aid penetration, tame recoil allowing spotting your own shots and the potential for exceptional accuracy are all part of the ARC package.

Initial Hornady load data was developed in a Noveske N4 AR-15 with an 18-inch barrel and 1:7.5 rifling twist. Top velocities with 108- and 110-grain bullets reached 2,575 fps. “Gas gun” data includes 52,000 psi maximum pressures. Bolt-action data was developed in a CZ-USA Model 527 with a 24-inch barrel and 1:7.5 rifling. Maximum pressures in the turn bolt were established at 62,000 psi. That, and the longer tube, resulted in average of 300-fps velocity gains.

Handloading data for the 6mm ARC was newly minted at the onset of this month’s-long project. Hornady’s initial “gas gun” data included only 108-grain ELD Match and 110-grain A-Tip Match bullets; lighter bullets from 58 to 105 grains were added within a couple of months. “Bolt gun” data appeared simultaneously with “gas gun” light-bullet data, listing bullets from 58 to 108 grains. Hornady also supplied dies, brass and initial factory ammunition for this project. As of press time, Hornady remained the primary source for properly head-stamped brass.

Since the 6mm ARC was designed to run heavy 6mm bullets – with a fast 1:7.5 twist necessary for proper stabilization – I saw no reason to dabble in lighter bullets from the AR. Powder choices include medium-burn rates used in cartridges such as the .224 Valkyrie and .22 Nosler. Most options provided in current Hornady data are modern, clean-burning formulas, most temperature-insensitive and/or including copper-erasing formulas. ARC brass uses small rifle primers, and Federal Premium’s Gold Medal AR Match were used for all AR loads.

AR test loads included Berger’s 108-grain Elite Hunter with a .559 G1 BC, 110-grain Hornady A-Tip (.604 G1 BC) and Nosler’s 115-grain RDF with a .634 G1 BC (the highest BC 6mm bullet I’m aware of). Hodgdon’s CFE 223 and LEVERevolution (LVR), Accurate’s A-2520 and Alliant Power Pro 2000-MR and Reloder 15 were chosen because they offer top velocities in all bullet weights.

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