CMMG Endeavor 6mm ARC Rifle – New Long Range AR-15 Cartridge ~ VIDEO

Duncan reviews the CMMG Endeavor 6mm ARC Rifle in the all-new Long Range AR-15 6mm ARC Cartridge.

U.S.A.-( CMMG has always produced impressive rifles and constantly pushed the limits of what the AR-15 can provide to the consumer market, and were one of the first companies to jump on the 6mm ARC, a new cartridge for the AR-15. The CMMG Endeavor quickly proved that this cartridge was exactly as promised.

CMMG Endeavor Rifle chambered in 6mm ARC caliber
CMMG Endeavor Rifle chambered in 6mm ARC caliber

When the 6mm ARC (Advanced Rifle Cartridge) was announced by Hornady earlier this year it definitely caught my attention. Designed for a group within the U.S. DoD, the 6mm ARC promised to deliver better performance at longer ranges than the 5.56 and would function reliably from an AR-15. With bullets weighing in at 108gr it will deliver more energy on target than the .223 and still be comparable to the energy found in the larger 308. Long-range performance, in a lightweight rifle, with a powerful downrange effect; it sounds too good to be true.

Hornady BLACK 6mm ARC 105 gr BTHP
Hornady BLACK 6mm 105 gr BTHP

Hornady 6mm ARC – Comparisons, & Purpose

If you are curious about the origins of the 6mm ARC, here is a link to a great article by Gun University that goes into much more information on the cartridge. The overall gist of this cartridge is to think of it like an updated 6.5 Grendel. Both cartridges use the same bolt head and magazines, but the 6mm ARC has a better trajectory than the Grendel. While the two cartridges share similarities the more important comparisons are between the 6mm ARC and .223/5.56.

.223 (Left), 6mm ARC (Center), .308 (Right)
.223 (Left), 6mm ARC (Center), .308 (Right)

When you look at the 6mm ARC it is interesting to consider what the .223 cannot do. First off the .223 is not going anywhere, its basically impossible to dethrone the king, but it is far from perfect. Long range accuracy is well known to be the drawback of the .223. On the other side, 6mm ARC drops less and is less susceptible to wind drift than the smaller .223/5.56 bullet. From my own time with the CMMG Endeavor, I was able to get hits in windy conditions at 900 yards. This is largely due to the better BC found in 6mm bullets.

While the 6mm ARC has more recoil than the .223, it has considerably less than a .308. It is still feasible to perform two quick shots with the 6mm ARC, and the rifle itself is not too heavy to make offhand shooting difficult.

Recoil on the 6mm ARC is not harsh, and it is still easy to make fast follow up shots.
Recoil on the 6mm round is not harsh, and it is still easy to make fast follow up shots.

It is also very easy to assemble an upper to accept the 6mm ARC and you can use an AR-15 lower that you already have laying around in your safe. All you need is a 6mm ARC barrel, a 6.5 Grendel bolt, and a 6.5 magazine; that’s it. The 6mm ARC makes it incredibly accessible for you to have a 1000 yard rifle in your gun safe. The cartridge is very impressive and was matched by the rifle provided by CMMG.

Energy comparison between .223, 6mm ARC, and .308. Infographic provided by Hornady.
Energy comparison between .223, 6mm ARC, and .308. Infographic provided by Hornady.
Trajectory comparison: 6mm ARC (Red Line). Infographic by Hornady.
Trajectory comparison: 6mm ARC (Red Line). Infographic by Hornady.
Velocity comparison: 6mm ARC (Red Line). Infographic by Hornady.

CMMG Endeavor in 6mm ARC

CMMG’s Endeavor 300 MK4 rifle (also see the CMMG Endeavor Uppers in 6mm ARC for do-it-yourself types) has all the features that you would expect from a high-end precision rifle. With a 20” barrel, the MK4 harnesses much of the available velocity from the ARC. During my time with the rifle, I noticed a very steady and reliable ejection pattern and did not encounter any malfunctions. The inclusion of a Magpul PRS stock, Geissele Automatics SSA 2-Stage trigger, and ambidextrous safety made the rifle a top-notch precision firearm. One thing that surprised me was how light the rifle was, even with a Primary Arms SLx 4-14x44mm FFP MIL-DOT Riflescope and Magpul bipod. This was one of the big factors for the 6mm ARC’s creation, it saves a lot of weight compared to an AR-10 platform.

Out on the range, the CMMG Endeavor and 6mm ARC were everything I hoped it would be. I found a ballistic chart for the Hornady BLACK 6mm ARC 105gr BTHP ammunition that I was shooting. Using the elevation drop on the chart below I found it was dead accurate while using the Primary Arms SLx 4-14x44mm FFP MIL-DOT Riflescope. At 400 yards the 6mm ARC only dropped 2.0 mils, and every distance below that was also accurate out to 1000 yards.  Even while shooting on the windy foothills of Colorado I noticed very little shift with the cartridge.  If you have a look at the video on our YouTube channel you can see for yourself that 6mm ARC sends a lot of energy downrange, leaving some smaller targets spinning full revolutions around its supporting bar.

Hornady BLACK 6mm ARC 105 gr BTHP Ballistics, chart courtesy of Gun University.
Hornady BLACK 6mm ARC 105 gr BTHP Ballistics, chart courtesy of Gun University.
The CMMG Endeavor has all features you want on a precision rifle, and the 6mm ARC was more than capable of hitting targets out to 1000 yards.
The CMMG Endeavor has all features you want on a precision rifle, and the 6mm ARC was more than capable of hitting targets out to 1000 yards.
CMMG Endeavor 6mm ARC
CMMG Endeavor 6mm ARC

If there is one thing the new 6mm round really needs it is a dedicated magazine. Using the 6.5 Grendel magazine is good enough for now, and I experienced no issues with the 10 round E-Lander I purchased, or the two provided magazines that came with the rifle which I believe were C-Products with CMMG labeling.  While they functioned reliably, I think a dedicated magazine would provide better capacity and would only help further reliability. I shot roughly 300 rounds through the rifle provided and had no issues throughout that process, but a dedicated magazine would only see that performance improved.

6mm ARC uses 6.5 Grendel magazines.
The ARC uses 6.5 Grendel magazines.

The search for the “perfect cartridge” is one that has spanned for decades now. Manufacturers have long sought the power of the .308 while having the advantages of the low-recoil found in .223/5.56. Combining those two features while maintaining accuracy at longer ranges has, until now, required a heavier firearm like the AR-10. Hornady’s recent release of the 6mm ARC is promising to change that and deliver a cartridge that can easily reach 1,000 yards while still functioning in an AR-15. CMMG has that AR-15 in the Endeavor.

.223/5.56 AR-15 BCG (Left) 6mm ARC AR-15 BCG (Right)
.223/5.56 AR-15 BCG (Left) 6mm AR-15 BCG (Right)
CMMG Endeavor 6mm ARC
CMMG Endeavor in the new cartridge


  • CALIBER: 6mm ARC
  • BARREL: 20”, 1:7.5 twist, 6mm ARC, MT 416SS, SBN
  • MUZZLE: MUZZLE: SV Brake, threaded 5/8-24
  • GAS PORT LOCATION: Rifle Length with SLR Adjustable Gas Block
  • RECEIVER: Forged 7075-T6 AL M4 type upper, AR15 type lower
  • HAND GUARD: CMMG RML15 M-Lok hand guard
  • FINISH: Cerakote Receivers and Hand Guard
  • CHARGING HANDLE: CMMG Oversized Ambi
  • BUTT STOCK: Magpul PRS
  • TRIGGER: Geissele Automatics SSA 2-Stage
  • MAGAZINE: 10rd
  • WEIGHT: 9lbs 2oz(unloaded)
  • LENGTH: 40.5”

See AmmoLand News’ related review of the CMMG Endeavor 300 Series in 6.5 Creedmoor found here.

About Duncan Johnson:

Duncan is a firm believer in the Second Amendment and that “shall not be infringed”, means exactly that. A life-long firearms enthusiast and a graduate of George Mason University, now competing regularly in 3 gun competitions, Duncan is always looking to improve his shooting skills. Duncan is a regular contributor to AmmoLand and assists in the everyday gun-news publishing as an assistant editor.

AmmoLand Editor, Duncan Johnson about to go "full semi-auto" at Independent Studio Services

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What about barrel heat?? It is my understanding that the 6mm and 7mm bullets have a lot more contact with the barrel, which causes them to heat up quickly, which then affect accuracy and barrel life,


The “real” comparison, to me, is the 6.5 ARC AR-15! Not showing either the 6.5 Grendel & 6.5 ARC velocities, energy, & drop, really hurts the impact of this article. Also, it’s still the 16” barrell that we want to see for comparisons


Earlier reports used 108gr ELD loads. To me at least, the groups fired on paper (with 7.7 and 7.5 twist barrels) showed signs of not being adequately stabilized. Running that bullet through the Berger stability calculator suggested a 1:7 twist to adequately stabilize it. Most reviewers expressed surprise that lighter bullets shot better. I have to wonder why Hornady and the participating rifle manufacturers opted for the slower twist barrels or whether they based their decisions based upon testing with lighter bullets?




Has the military decided on a new round and weapon yet? I thought they were testing the 6.8mm rounds? Rather have what military will be using, makes ammo easier to come by when the SHTF… God bless our troops, vets, and law officers.


They are testing the 6.8 for their new Squad Automatic Weapon at least that is my understanding.

Matt in Oklahoma

The more yall advertise this cartridge the more suspicious I become.
A good thing doesn’t need pushing