The 6mm ARC Cartridge: Why the Military Needed It

All anyone can say about the identity of the military unit that adopted the 6mm ARC (even before the round’s public announcement) is that the unit is “a notable Department of Defense entity.” Needless to say, that doesn’t mean the entire U.S. Army, Navy or Marine Corps has adopted the 6mm ARC — and they likely never will. This cartridge could only have been adopted by someone in the Special Operations community for good reasons.

Lessons from the battlefield have taught our soldiers the benefit of being able to hit targets as far away as possible, and no one does that as well as we do. No nation in the world produces riflemen like ours because nowhere else in the world is the citizenry so freely armed. This makes it possible for America’s youth to grow up shooting and acquiring the skills they’ll need on the battlefield from an early age. The armed citizenry also serves as a huge repository of marksmanship skills that are currently on display at any Precision Rifle Series (PRS) match. This allows our military to field soldiers who have the knowledge and understanding required for effective and accurate small arms fire at extended distances. This doesn’t mean everyone in a Special Operations unit is a precision-rifle ninja (because that’s not true), but all it takes is one or two guys who are for the entire unit to become very good, quickly.

Until now, an issue that U.S. soldiers faced was that the cartridges in our military’s inventory did not lend themselves well to precision long-range fire from the issued carbine or semiautomatic sniper rifle. The 5.56mm NATO is a poor choice for anything past 300 yards, and moving to the .308 Winchester only offers a marginal improvement in ballistics while coming at the cost of a significant weight increase in both firearm and ammunition. The 6mm ARC exists (and will thrive) because it solves this problem, and now the citizenry can benefit from the military’s solution.

The Problem

America’s military has been at war against terror for 20 years. We have certainly learned a lot along the way, but so have our enemies. For example, the Taliban in Afghanistan knew that most of our guys shoot carbines chambering 5.56 NATO; if they stay 600 yards away, there’s no need to fret small arms fire much.

The unit that adopted the 6mm ARC decided that they still wanted to get good hits at 600 yards and beyond, so they started issuing more rifles chambered in .308 Winchester because it is capable of that kind of performance. The problem is that the rifles and ammunition are both heavier than their 5.56mm NATO counterparts.

Before the “hit the gym, cupcake!” crowd shows up, I’d like to point out that the members of this unit are probably the most physically fit human beings in the military. It’s important to remember that additional weight slows everyone down (regardless of fitness level) and speed on the battlefield is vital.

One of the mission types I participated in years ago was called an “offset.” We’d insert by helicopter far enough away from the target that people on the target wouldn’t get nervous. We’d then hustle a few kilometers to our meeting where we could handle business. Carrying excess weight on these missions increased physical pain for all involved and delayed successful mission completion. We devoted a lot of effort to minimizing weight, and that tradition continues today.

Why Not the 6.5 Grendel?

An early candidate for this unit’s new cartridge selection was the 6.5 Grendel (2003). It keeps the firearm weight low since any AR-15-style rifle can be chambered in it, and ammunition isn’t prohibitively heavy. Where one round of 77-grain 5.56mm NATO weighs 13 grams, a round of 123-grain 6.5 Grendel weighs 18 grams. Comparatively, one round of 175-grain .308 Winchester weighs 26 grams.

While the guns and ammo were nice and light, the ballistics of the 6.5 Grendel were good, but not great. In the “plus” column were the high ballistic coefficient (BC) and decent muzzle velocity that kept a 123-grain projectile supersonic out to 1,150 yards in a standard atmosphere. Six-five bullets that were tested against intermediate barriers such as windshield glass also performed well. The bullets stayed together even when impacting barriers at muzzle velocity.

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In the negative column for the 6.5 Grendel were its short-range terminal effects, which is a big deal when considering the cartridge’s intended application. There’s been a lot of speculative chatter about the external ballistics of the 6.5 Grendel versus the 6mm ARC on the interweb, but the 6mm ARC wins in every category. The question lingers about whether or not the increased performance is worth re-barreling a rifle.

Almost no consideration (outside military circles) has been given to the terminal effects offered by each cartridge. Here, there is a significant difference in how the cartridges perform.

The U.S. Special Operations community has been using match bullets for precision rifle application for decades. About 15 years ago, they realized that putting a polymer tip on a match bullet was a great way to ensure more uniform BCs and, therefore, increase hit probability on targets at extended ranges. (That’s how the legal justification is written.) The use of a polymer tip had the incidental benefit of greatly increasing any bullet’s terminal effects because polymer-tipped bullets begin expanding and fragmenting on contact. Traditional open-tip match (OTM) bullets require 6 to 8 inches of penetration before they do anything, and that’s at muzzle velocity. By the time an OTM bullet is out at 300 to ­400 yards, there is little or no expansion and terminal effects greatly diminish.

A good first step to develop an ideal cartridge for Special Operation’s was to start with the terminal performance of the 6.5 Grendel, and then develop a baseline for further evaluation. The minimum velocity required for bullet expansion and fragmentation on most polymer-tipped bullets is 1,800 feet per second (fps). I chronographed a 123-grain polymer-tipped 6.5 Grendel bullet out of an 18-inch barrel at 2,451 fps. That 123-grain bullet will maintain a velocity above 1,800 fps out to 400 yards in a standard atmosphere. (Feel free to check my work on any ballistic calculator.)

Behold, the 6mm ARC!

The ballisticians at Hornady are no dummies. They quickly realized that shrinking the diameter of the bullet would allow them to lower projectile weight while boosting muzzle velocity and, perhaps, increasing BC. Two calibers they looked at were .22 and 6mm.

There are two issues with .22-caliber bullets. The first is that they offer very little “splash” when a guy misses a target. If the shooter can’t see where the round impacts, it’s impossible to correct from it. The second problem is that they are small and moving fast, so any barrier they encounter is almost certainly going to destroy them. A fast-moving .22-caliber bullet performs poorly once it encounters a windshield.

The 6mm bullets Hornady tested at the higher velocities could be made to meet a wide number of performance parameters. By simply necking down a 6.5 Grendel case to 6mm and pulling the case’s neck-shoulder junction back .030-inch closer to the case head, space could be freed up for Very Low Drag (VLD) bullets. In this way, Hornady created the 6mm ARC.

Hornady designed the 6mm ARC around their 108-grain ELD-M bullet, too. Overall length fits fine in an AR-15 magazine, and the case is short enough to keep the bullet’s bearing surface above the neck-shoulder junction. This allows the entire case to be used for powder capacity.

Hornady’s case can push higher-BC bullets to greater velocities than the 6.5 Grendel. Where the 6.5 Grendel keeps a polymer-tipped 123-grain bullet above 1,800 fps out to 400 yards, the 6mm ARC can keep a 108-grain bullet above 1,800 fps out to 580 yards. Those velocities are from the same 18-inch barrel lengths for both the Grendel and the ARC. The 6mm ARC’s terminal performance window is 45 ­percent greater than the 6.5 Grendel’s. I didn’t test lighter bullets in the 6.5 Grendel because I wasn’t impressed with the BCs they offered. Of course, bullets are still lethal at velocities slower than 1,800 fps, and the 6mm ARC continues to outperform the Grendel in this metric. Where the 123-grain Grendel bullet stays supersonic out to 1,150 yards, the 6mm ARC is good until 1,350 yards.

To date, there are only three factory loads for the 6mm ARC, and they’re all from Hornady. There is the 108-grain ELD-M, 103-grain ELD-X, and the newest 105-grain BTHP from the Black line.

I tested these loads in two rifles: Seekins Precision DMR with a 20-inch barrel and a Wilson Combat Recon Tactical with a 16-inch barrel. I’ve created a table that shows the accuracy and velocities for each load in these rifles. The bottom line is that the 4-inch difference in barrel length accounts for about 100 fps in muzzle velocity. The Wilson Combat rifle came with an adjustable gas block, so throwing a short suppressor on it would make for a fantastic deer and antelope rifle.

Not Just for AR-15s!

While Hornady designed the 6mm ARC for the AR-15­plaform and U.S. Special Operations, it is already one of my favorite bolt-action rifle cartridges. I am not alone in my new love for small-cased 6mm cartridges. There is currently an explosion in the use of cartridges like the 6mm Dasher, 6mm BR, 6mm BRA, 6mm BRX, etc.

These cartridges dominate the PRS circuit because they can push high-BC bullets at high velocities while offering very little recoil. Minimizing recoil is important to these competitors because shooters can watch where their rounds impact and correct immediately for a follow-up shot.

When Hornady developed the 6mm ARC, they set maximum chamber pressure at the low level of 52,000 psi. The reason being is that the cartridge was designed primarily for an AR-15 with an enlarged bolt face. Making more room on the bolt face took away some of the material that surrounds the case head, so higher pressures would likely lead to premature wear and breakage of the bolt lugs that sit on either side of the extractor.

A fringe benefit of loading to such mild pressures is long barrel life. Hornady expects 6mm ARC barrels to last as long as a .308 Win.-chambered barrel.

The 6mm ARC is going to be ideal for dinging steel out to 1,350 yards or so, too, because it stays supersonic the entire time. The case was designed for VLD bullets and, when loaded to a more traditional rifle cartridge pressure of 64,000 psi (such as the 6.5mm Creedmoor), that same 108-grain ELD-M is going to leave the muzzle somewhere between 2,850 and 2,900 fps. Between the small case head, low bolt thrust (and recoil), and great velocity and BC, that’s some serious, long-range steel-dinging medicine.

I don’t expect there to be much, if any, factory ammo loaded at those pressures, but Hornady will develop bolt-gun reloading data for the 6mm ARC. Even when shooting the mild factory loads in a 24-inch barreled bolt-action, I’d still expect muzzle velocity to sit somewhere around 2,800 fps. For the record, many of the top-10 PRS shooters are launching the 110-grain A-Tip Match bullets at 2,750 to 2,800 fps. That’s fast enough for great ballistics, but so low-recoiling that a shooter can watch a bullet fly to the target. Of all the 6mm cartridges currently in favor with the competition crowd, the 6mm ARC is the only one offering those ballistics from inexpensive factory ammunition. Everyone else has to handload.

To top it off, the 6mm ARC will match those ballistics with less recoil because of its reduced case-head diameter and lower bolt thrust. Where the Dasher/BR crowd has a case head size of .473-inch, the 6mm ARC sits at .441-inch.

The 6mm ARC is now my favorite 6mm cartridge for all the reasons I’ve listed. I would expect loaded ammunition to become available in the near future with bullet weights varying from around 60 grains to 108 grains. (Handloaders can do this now if you have the components.) That’ll make the 6mm ARC good for varmint shooting, deer and antelope hunting, as well as competitive rifle shooting. No other factory-supported cartridge offers so much flexibility.