Hornady Offers New 6mm ARC Cartridge for AR15 Platform

A decade-and-a-half ago, our friend Robert Whitley created the 6mmAR, a wildcat based on the 6.5 Grendel necked down to 6mm. The great thing about the 6mmAR cartridge is that is offers superb accuracy with high-BC 6mm bullets, while working in AR15-platform rifles. The 6mmAR feeds and functions using 6.8 SPC or 6.5 Grendel AR magazines. We provided a full report on the 6mmAR way back in 2006.

Hornady Introduces the 6mm ARC Cartridge for AR15 Platform
Now Hornady, with no apparent credit to Whitley’s 6mmAR, has announced a copycat cartridge, which it calls the 6mm ARC (Advanced Rifle Cartridge). Hornady will offer loaded 6mm ARC ammunition and also cartridge brass. In addition Hornady will produce reloading dies, shell plates, modified case, and other accessories. SEE 6mm ARC SAAMI Specification.

Hornady states: “Hornady® is proud to introduce the new 6mm ARC (Advanced Rifle Cartridge). Recently tested, selected and fielded by a specialized group within the U.S. Department of Defense for its multipurpose combat rifle program, the 6mm ARC is a truly versatile cartridge that maximizes the potential of the AR-15 platform.” Hornady Ballistician Jayden Quinlan adds: “We subsequently modeled and tested a variety of designs in different calibers until we were able to produce the most flexible cartridge possible within the limits of the AR-15 system. The 6mm ARC does much of what larger cartridges can and everything that smaller cartridges can’t.”

NOTE: The rim diameter (0.441) and thickness of the 6mm ARC appear to be identical to the 6.5 Grendel as illustrated on page 40 of the SAAMI Industry Standards. From all we can tell, in other dimensions as well, this is a very close copy of Robert Whitley’s 6mmAR, a 6-6.5 Grendel wildcat. For example, 6mm ARC case length is 1.49″ vs. 1.51″ for 6mmAR. The Guns&Ammo report on the 6mm ARC states: “The 6mm ARC is based on the 6.5 Grendel. Hornady started by necking down the cartridge to 6mm and then pulled the shoulder back closer to the case head .030-inch. … Hornady also set maximum chamber pressure low at 52,000 pounds PSI. This slows extraction and makes it much less violent, prolonging the bolt’s life.”

There is an extensive discussion of the 6mm ARC on the Gun University website. You’ll find dimensions and performance numbers for the new cartridge along with videos showing the 6mm ARC cartridge tested in a Barrett AR-type REC7 DI rifle. Read Gun University Story HERE.

More Velocity Than a .223 Rem, Less Weight and Recoil Than a .308 Win
Hornady says the 6mm ARC “delivers substantially better ballistics than the [.223 Rem] and offers a much lighter gun/ammo system with 30% less weight than the AR-10/308 Win system. This new cartridge delivers less felt recoil than larger short-action-based cartridges, yet takes full advantage of modern, heavy-for-caliber 6mm bullets that provide excellent accuracy and ballistic performance at extended range.”

You can see a ballistic comparison between 6mm ARC, .223 Remington, and .308 Winchester in this Hornady chart:

Notably, a large number of AR gun-makers have committed to producing rifles or uppers chambered for the 6mm ARC. Some of these are: Adams Arms, Barrett, Brownells, Christensen Arms, GAP, Howa, Mossberg, Noveske, Odin Works, Radical Firearms, and Wilson Combat. In addition Geissele and CMC Triggers are listed as suppliers on Hornady’s 6mm ARC webpage.

The Proven Lapua Brass Alternative — 6mmAR and 6mmAR Turbo

If you want 6mm ARC performance in your AR15 with superior case life and consistency, consider doing the 6mmAR or 6mmAR Turbo instead. These are proven designs with winning records in high power competition. Redding produces dies, and complete uppers (with proper Grendel-sized bolt face) are available from 6mmAR.com.

Superior Accuracy with Impressive Ballistics
Robert explained to us: “Lapua’s Grendel brass is of exceptional quality, very hard at the back, with a well-annealed neck area, and it has a small primer pocket and small flash hole. When you start with this kind of brass, you have all the ingredients of great accuracy. Necked down to 6mm, it is like a long-bodied 6 PPC, that feeds and shoots well out of an AR-15. This gives you a case that can handle high pressures, with the accuracy pedigree of the 6PPC. It doesn’t get much better than that.”

To create the 6mmAR simply neck-down parent Lapua 6.5 Grendel brass, which has small primer pocket and small flash hole. Case capacity is 36.0 grains of water, filled to over-flowing. (For comparison the 6mm PPC holds about 32.5 grains). To prepare cases, simply run the 6.5 Grendel brass through a Redding 6mmAR full-length sizing die. The FL die’s internal neck diameter (0.267″) is spec’d to deliver about .004″ tension on the bullet–just right for a gas gun. Redding’s 6mmAR dies were built to Whitley’s specs and closely match the actual dimensions of the Lapua brass. This video from Robert Whitley shows how to form 6mmAR cases from Lapua 6.5 Grendel brass.

Velocities: With the recommended medium-burn-rate powders, bullets in the 85- to 90-grain range can be pushed to the 2800 to 2850 fps range. In most 6mmAR rifles, the 105- to 107-grain bullets can be driven to velocities of 2700 to 2750 fps in a 24″ or longer barrel. More speed is possible, but don’t count on it with the average barrel. If you want more speed the 6mmAR Turbo offers additional case capacity. This allows the 6mmAR Turbo to drive the heavier 6mm bullets at velocities close to 6mmBR Norma cartridge levels.

Magazines: The correct magazines are the key to a reliable, competition-worthy system. For best results with the 6mmAR or 6mmAR Turbo, use special 6.8 SPC mags or Alexander Arms Grendel mags. Those specialized mags will provide better reliability than standard .223 Rem/5.56 mags for the AR15.