6mm ARC Carbine: Juice Worth The Sqeeze? 

Almost every gun owner has the same thought for almost all of their guns at one time or another. From the 10/22 plinker to the shotgun on the wall to the AR under the bed. “What if I have to use this in a gunfight?”

Admittedly, as someone who has never been in a gunfight and would rather not have that life experience — for almost all of us the thought is academic. But it can still be a useful thought, even if you’re trying something new.

6mm ARC quietly landed during the pandemic in 2020. Introduced by Hornady with a small press release and almost no fanfare, it didn’t get the close look that it deserved until much later.

Normally when a new cartridge is released and it isn’t an instant smash hit, it dies quickly and quietly. 

But 6 ARC had an ace up Hornady’s sleeve. 6 ARC didn’t rely on you and me to buy it because 6 ARC was specially designed and manufactured for the Department of Defense. 

Who exactly in the DOD, Hornady has never said, officially. But keen-eyed observers can find pictures of 6 ARC rifles made by Gisessle in the hands of Navy SEALS and other branches of SOCOM.

I love a good long range cartridge and I love the AR-15, so 6 ARC had my interest from the start. I’ve toyed around with it in the years since its release with a Uintah Precision bolt-action upper on my AR and really enjoy the round. But 6 ARC really is designed to be semi-automatic. And I wanted to know what it can really do.

Thus started an almost year-long project to build a “fighting carbine” version of a 6 ARC AR-15. This isn’t a clone of what green-eyed members of the DOD are running. This is just a “pistol” designed to take advantage of the strengths of what 6 ARC can do, both at close range and at long range, even if its only targets will be cardboard and steel.

BUILD LIST

BALLISTICS

77gr Blackhills 5.56 NATO 16″ Barrel

108gr 6 ARC 14.5″ Barrel

6 ARC is ballistically much better than 5.56 NATO. From my rifles, 77gr Black Hills 5.56 NATO runs about 2,675 FPS in a 16-inch rifle and has issues with repeated hits past about 600 yards. Doable, but hard. Same can be said for most 5.56 NATO loads at that range.

108gr 6 ARC from this Rosco barrel is pushing 2,445 FPS and the performance is pretty awesome.

Bullet drop is comparable between the two from 0-400 yards, only a couple inches different either way. Past that, 6 ARC pulls ahead but only a little. Even at 1,000 yards, the difference is only 495 inches Vs. 453 inches of drop. Not that big of a deal when you’re looking at the big picture.

The huge difference comes in velocity at range and power on target. This 5.56 load drops subsonic just past 800 yards while the 6 ARC is still supersonic past 1,000. KE on target is also almost double at 1,000 because of the larger bullet and greater retained speed.

5.56 NATO past 500 yards can get sporty real fast. 6 ARC past 500 yards is still almost point and click.

ON THE RANGE

6mm ARC is built for reaching out, but with a 14.5-inch barrel, this gun is still leaning more on the running and gunning side than on the long range side. While 6 ARC is better at distance than 5.56 NATO, it still doesn’t have the legs of .308 Win, 6.5 Creedmoor, or anything bigger.

To support this I went with a Lead & Steel Pandora PB-3 red dot on top of a Trijicon Credo 2-10×36 scope. 

Mounted using a BoBro scope mount with a KOFN ring mount for the Pandora (love this set up), this gives the best of both worlds. Hits on steel at 800+ yards were surprisingly easy, but running a 2-gun course with cardboard at 5 to 20  yards was also fast.

This was my first time running a BoBro mount and it won’t be the last. Rarely am I impressed with a scope mount, but BoBro outdid themselves. The stepped scope rings make for a rock solid locking system and the universal QD system is the first I’ve ever used that worked without adjustment across all rails I’ve tried.

The KOFN system is a dove-tail style mounting system that returns to perfect zero while allowing very quick detachment. Nice for when I want to change out what dot is on top or taking the dot off when I’m doing dedicated long range shooting.

Combined with the Dead Air Lazarus 6 suppressor and this was not only a quiet shooter but a very well-gassed one as well. Something 6 ARC owners have quickly found out is that this cartridge can be very gassy when suppressed without a way of controlling the gas like an adjustable gas block. Personally, I didn’t have this problem because the Lazarus 6 has some outstanding gas flow design to it.

A Cloud Defensive Rein 1.0 on a BoBro KOFN mount helped enhance the close-up abilities while an MDT Ckyepod on a 1913 Picatinny rail gives the perfect option when needing to reach out. The KOFN mount in this case was nice because it make moving the light from rifle to rifle a lot easier. Useful since I have too many rifles and not enough lights.

Overall, this build worked exactly as I hoped it would and even better than I expected. 6 ARC is able to reach out, but it doesn’t come with too much recoil to prevent being able to go fast and close when needed. One of the best strengths of 5.56 NATO is the ability to be a do-all cartridge most of the time, but leans towards shorter distances. 6 ARC leans the other way and shines past at least a couple hundred yards. 

It can still do work up close, but it’s hard to ignore the fact that it’s more comfortable with a little distance.

AMMO, MAGAZINES, AND MORE

Feeding the 6 ARC is likely the hardest part of trying to run it. Between cost and availability, it hurts the wallet when you need 150 rounds per match at least. Handloading 6 ARC is pretty easy with nothing weird that might trip you up and load data to get you started. 

Handloads with MidwayUSA 105gr soft-pound factory seconds helped keep the price down for easy-to-hit targets while anything that required nice ammo was mostly solved with Hornady 6mm ELD Match provided by Hornady.

Magazines are another nightmare all their own. 6 ARC uses Grendel mags, but Grendel mags aren’t perfect for 6mm ARC. Dedicated 6mm ARC mags are available from Geissele for $100+ each or from ASC and Duramag for more reasonable prices.

I also tried Amend2 magazines but getting one to feed wasn’t easy.

In the end, Duramag won out for reliability with only 1 malf across about 250 rounds. ASC never finished a mag without failing and Amend2 would normally have a misfeed at least once every third reload.

One downside to 6 ARC is the fact that it takes Grendel Type 2 bolt faces. This pistol used a Cryptic Coating BCG that performed flawlessly and was magically easy to clean. That said, even the best BCG can’t overcome the fact that Grendle Type 2 bolt faces in an AR-15 are a bit thin. Just not as much meat on the bone as you might hope for.

With 6 ARC being more of a long-range cartridge than other Grendel bolt face options in the past, a lot of people are trying to get the most muzzle velocity possible, and with that comes extreme pressure. That pressure is leading to a lot of cracked or broken bolt faces.

There are some interesting options on the market to help address this, but my solution was to dial down my handloads to keep pressure reasonable.

GREAT BARREL, GREAT GROUPS

I first got to see the new Rosco Purebred 6 ARC barrels at SHOT last year. Almost a year later now, this is technically my second barrel. Rosco rushed a barrel for me after SHOT so I could start this project. Unfortunately, that barrel had a chamber issue and needed to be sent back. Bummer too since the precision was great.

Rosco’s second barrel has performed perfectly. Sub-MOA with Hornady ELDM ammo at 100 yards and flawless reliability. Thicker profile than most standard barrels, the extra meat helped to control heat during long strings of fire. This comes in really handy after blasting a magazine worth of rounds at close cardboard before transitioning to steel targets 400+ yards away.

LOOSE ROUNDS

6 ARC isn’t the perfect cartridge for everyone, but it does do some things better than what 5.56 NATO can do. If I was building this gun again, I would have gone with a 16-inch barrel to avoid the legal pitfalls of having to keep it a pistol. Or I would have gone 12.5 inches and registered it as an SBR. Either way, I feel like 14.5 inches wasn’t the ideal length and I could have chosen better.

That said, the barrel performed really well. From 5 yards to 800 yards, this gun just worked. 6 ARC is a laser when you let it.

I won’t be replacing all of my 5.56 guns with 6 ARC, but I definitely see myself playing with 6 ARC more in the future. After building a small taste of what the spooky boys in DOD are playing with, it’s easy to see why Hornady was tasked with designing a game-changer.