October 01, 2020
Rifle & Optics Editor Tom Beckstrand introduced Hornady’s 6mm Advanced Rifle Cartridge (ARC) to the Guns & Ammo’s digital audience with a YouTube video that immediately went viral on June 3, 2020. This was a carefully kept secret as the cartridge had been developed, standardized and adopted by a notable unit within the Special Operations community. This occurred before the existence of the cartridge was made public. For that unnamed unit, we’ve only been told that “the 6 ARC is a replacement for the 7.62 NATO for combat operations.” This is the second time that the announcement of a new cartridge was preceded by the military’s adoption; The first was Hornady’s .300 PRC announced on October 16, 2018.
Our enemies in the War on Terror have learned that they can escape the effective range of the 5.56 NATO round if they keep a 500- to 600-meter buffer, which isn’t difficult in the arid desert terrain of the Middle East or the jagged mountain ranges across Afghanistan. To meet these challenges, a number of military units have employed 7.62/.308-based rifles such as the FN Mk 17, HK 417, Knight’s Armament Mk 11 and M110 series, as well as a few others. However, we must not forget that the weight penalty of carrying these heavier guns and a combat load of ammo is a burden on our troops. Minimizing the weight they carry is always a focus of procurement specialists, which also makes logistics more economical and efficient.
Why didn’t the Department of Defense (DoD) select another cartridge already in existence? For example, the 6.5 Creedmoor is the most popular rifle cartridge sold commercially and is easily capable of ranges beyond 600 meters. The 6.5 Creedmoor is based on the 7.62/.308 case, which means semiautomatic-rifle platforms would still be the same size as the AR-10/SR-25 platforms, and ammo weight would be similar. The same answer applies to other cartridges such as the .260 Remington. Plus, the velocity of the Creedmoors are faster, which means they have a shorter barrel life than the 6mm ARC. A 6.5 Creedmoor barrel typically lasts between 2,000 and 3,000 rounds, while the 6mm Creedmoor experiences serious velocity drops starting between 1,500 and 1,800 rounds. You can expect the barrel life of the 6 ARC to last 5,000 rounds (similar to a 7.62 NATO), in part because it’s burning less powder.
How does the 6 ARC best the 6.8 SPC or 6.8 SPC II? The 6.8 SPC was an improvement over the 5.56 NATO developed in 2002, and was introduced to SAAMI by Remington. Though the 6.8 achieved better terminal ballistics at close-quarter distances than the 5.56 NATO, it was never effective out to 1,000 yards. At close ranges, the 6.8 SPC is great for hunting and personal defense, but few major companies still support the cartridge due to a lack of demand.
Why wasn’t the .220 Russian or 6 PPC selected? Invented in the 1950s, the 5.6x39mm is a 7.62x39mm cartridge necked down to accept a 5.6mm bullet. SAKO and Lapua introduced it to the U.S. as the .220 Russian in 1965. It became the parent case for several cartridges, including 6mm PPC and the 6.5 Grendel, the latter of which the 6mm ARC was designed on.
Since it was introduced as a competition cartridge in 1975, the 6mm PPC has developed a reputation for being one of the most accurate and successful short-range benchrest cartridges. Unfortunately, the 6 PPC, and other benchrest cartridges such as the 6 BR and 6 Dasher, are short-range wildcats for competition use. With such non-standard wildcats, dimensions vary.
And the .24 Nosler? The .24 Nosler, when loaded with a longer bullet like a 105-grain boattail, exceeds the overall length limitations of an AR-15 magazine. The 6mm ARC will accept 108-grain bullets, as well as heavier ones such as 110s without the same magazine compatibility issue.
Why not the .224 Valkyrie? Federal necked down a 6.8 SPC case to accept a .22-caliber bullet in 2017 to create the .224 Valkyrie. It managed to fit cartridges loaded with very fast 60-grain Nosler Ballistic Tips to 90-grain MatchKings with high ballistic coefficients (BC) in an AR-15-pattern rifle. However, the .224 Valkyrie experienced feeding and reliability issues. To add, the 6 ARC is easier to see hits and misses at distance than the .22-caliber bullet. The .224 Valkyrie offers many bullet options for varmints, but many states require a minimum of 6mm to hunt bigger game such as deer.
What about the 6.5 Grendel? In fact, the 6mm ARC is based on a 6.5 Grendel case, but there are differences. The 6 ARC is built on a necked-down Grendel case, but the shoulder of the case has been pulled back .0030 inch. These new dimensions mean that the 6mm ARC is better qualified to optimize Very Low Drag (VLD) projectiles that already exist for improved stability and range.
Benefiting from the Grendel
Basing the 6 ARC on the Grendel case meant that the accuracy potential could harness the benefits of the 6mm projectiles, but it also allows the use of readily available AR-15 bolts and magazines. Using the Grendel case as a basis for the 6 ARC’s design meant that there wouldn’t be feeding, extraction or ejection issues as there typically are when trying to get a new cartridge to run reliably in an AR-15 platform.
The 6.5 Grendel typically uses a heavier 123-grain projectile with a velocity of 2,580 feet per second (fps) out of a 24-inch barrel. (There are even 150-grain-class projectiles for the Grendel, but there’s a significant loss in velocity.) Hornady’s Match ELD-M bullet offers a BC of .506 (G1) and .255 (G7). I own an Alexander Arms DMR rifle in 6.5 Grendel and regularly see velocities using this bullet average 2,465 fps. It’s maximum-effective range tops out near 800 yards. The 7.62 NATO projectile slows to subsonic at a little more than 1,050 yards, while the 6.5 Grendel is subsonic just before 1,000 yards. The 6mm ARC doesn’t go subsonic until 1,100 to 1,200 yards when shot out of an 18-inch barrel, and 1,300 yards when fired from a 24-inch barrel.
The 6mm ARC was developed around a 108-grain bullet, specifically Hornady’s Match ELD-M. The BC for this bullet is .536 (G1) and .270 (G7), so it has the potential to maintain energy, velocity and wind resistance at long distance very well. The SAAMI chart indicates that the muzzle velocity is 2,700 fps, while Hornady produced 2,750 fps out of a 24-inch barrel. The lighter 103-grain ELD-X bullet in Hornady’s Precision Hunter load increases that velocity in the same barrel slightly to 2,800 fps. However, the 6mm ARC will service the military and civilian shooters more often in AR-15-type rifles, meaning barrel lengths will likely be closer to 18 inches. Guns & Ammo’s first sample rifle chambered in 6mm ARC was a Ruger AR-556 MPR with an 18-inch barrel. Using a LabRadar chronograph to measure velocities, we saw an average of 2,559 fps using the 108-grain Match load and 2,633 fps with the 103-grain ELD-X. The Match load lost 191 fps, and the Precision Hunter load lost 167 fps in a 6-inch-shorter barrel. During a separate test using LabRadar, Tom Beckstrand recorded a velocity of 2,698 fps out of a Seekins Precision DMR with a 20-inch barrel while shooting the 108-grain Match load.
SAAMI indicates that chamber pressure is set at 52,000 psi, which is mild and extends the potential barrel life, as well as the life of a rifle’s components. Beckstrand believes this has to do with the AR-15 bolt used to shoot the 6.5 Grendel. “The 5.56 bolt face is pretty beefy around the perimeter,” Beckstrand said. He added, “The 6.5 Grendel bolt face doesn’t have as much mass beneath the lugs, which means the two lugs flanking the extractor are going to be vulnerable to breakage if you hot-rod the cartridge.” The max pressure that SAAMI has set for the 6mm ARC will ensure that rifles and components last.
Troops will be able to engage the enemy in close quarters and engage threats with greater effectiveness out to 700 meters, and the cartridge doesn’t fall below subsonic velocities until beyond 1,000 meters.
At the Range
Since its introduction, the 6mm ARC has benefited from a lot of marketing and conversation. One of the first concerns was what guns were going to become available, but it quickly became obvious that Hornady had been collaborating with a long list of gun makers since it was SAAMI approved on January 20, 2020. Among them are Barrett, Brownells, Christensen Arms, Geissele, Howa, Mossberg, Proof Research, Uintah Precision and Wilson Combat. To evaluate the 6mm ARC in time for the public launch on June 3, Tom Beckstrand obtained a Seekins Precision DMR, and I ordered a new Ruger AR-556. Beckstrand’s review of Seekins’ DMR rifle in 6mm ARC is forthcoming, but I was joined by Associate Editor John Oller to develop performance data at 100, 200, 500 and 1,000 yards with Ruger’s AR.
The Ruger AR-556 featured a wonderful sub-4-pound trigger and an 18-inch cold-hammer-forged (CHF) barrel. Ruger selected a 1:7.7-inch twist rate for the 6mm ARC, the same as used in their barrels chambered for 6mm Creedmoor. At 550 feet above sea level during a 72-degree clear day, Oller and I observed slower-than-expected velocities averaging 2,560 fps for 108-grain ELD-M and 2,630 fps for the ELD-X. With a new Leupold Mark 5HD 5-25x56mm scope providing a clear view of our targets, we each managed several five-shot groups that measured near .75 inch at 100 yards. Unexpectedly, the 103-grain ELD-X hunting load shot tighter groups than the ELD-M! The best group was fired by Oller and measured .46 inch, while my best was .52 inch. I reasonably believe that Ruger’s AR-556 is capable of half-minute accuracy when paired with the right load. It’s a nice rifle.
However, there were a number of groups with flyers that opened some up to 1 and 1.25 inches. Usually four rounds would be tight with a fifth flyer elsewhere on the target. At 200 yards, groups averaged between 2 and 3½ inches. We attributed this to the ammunition’s inconsistent velocity observed earlier. Despite our previous experiences testing Hornady Match ammunition, the LabRadar measured an extreme spread (ES) of 63 fps and a standard deviation (SD) of 19.6 for 10 rounds, twice the variance we expect. The Precision Hunter load produced an ES of 41 fps and an SD of 14.2, which explained why we saw better groups downrange. We have ordered new lots of each load for further testing and have reported our findings to Hornady. If the velocity spread can be reduced, I suspect that flyers would be eliminated.
Still, the Ruger shot well and Oller continued to impress at 500 and 1,000 yards. I plugged in the velocity information to the Hornady 4DOF app on my iPhone and determined that we needed to come up 13.27 MOA (3.86 MRAD) at 500 yards and 43.42 MOA (12.63 MRAD) at a grand. For comparison, my Alexander Arms DMR in 6.5 Grendel with same-length barrel uses almost the same come-ups at 500 yards, but required a total of 45.76 MOA (13.31 MRAD) to touch steel at 1,000. Our best four-shot group at 500 yards measured 3.4 inches, while 1,000 yards yielded a 5.25-inch group. A fourth shot was attempted that struck the bullseye, but opened the group up to 12 inches. We are eager to revisit long-range shooting when we have a more consistent lot of ammunition on hand.
There is still a lot of testing we need to complete, but our first look has everyone involved excited. The 6mm ARC looks extremely promising, so much so that we’ve already ordered several other rifles and additional loads for testing. Though the 6mm ARC was developed to run in a gas gun, almost everyone wants to know how it shoots in a bolt gun. Though I believe 100-yard accuracy out of a gas gun will average between .75 and 1.25 inches for most, I see the 6mm ARC delivering half-minute groups from a small bolt-action given a sweet barrel. Regardless of platform, I’m willing to bet that the 6mm ARC will soon be formidable competition between 400 and 800 yards versus other cartridges.
Ruger AR-556 MRP Specs
- Type: Direct impingement, semiautomatic
- Cartridge: 6mm ARC
- Capacity: 27 rds.
- Barrel: 18 in., 1:7.7-in. twist
- Overall Length: 35 in. to 38.25 in.
- Weight: 6.8 lbs.
- Handguard: 15 in., free-float, M-Lok
- Stock: Magpul MOE SL, 6-position adj.
- Grip: Magpul MOE
- Finish: Type III Hardcoat anodized (aluminum); Melonite (steel)
- Trigger: 3 lbs., 13.8 oz. (tested)
- Safety: Two-position, lever
- Sights: None
- MSRP: $899
- Manufacturer: Ruger, ruger.com
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