When I heard rumors of a new 6mm cartridge from Hornady, I was intrigued. Yeah, these days we see a lot of “new” cartridges that just duplicate the ballistics of existing cartridges. They offer little more than a shiny new name and slick marketing. I’ve done my research and here’s what I’ve found comparing 6mm ARC vs. 6.6mm Grendel.

We can start here: Hornady has done a lot to advanced bullet and ammunition design. They solved some problems that plagued ballisticians and bullet designers for decades.

Why 6mm Rounds?

I’m also a big fan of cartridges firing bullets in the 6mm, 6.5mm and 7mm diameters (.243, .264, .284). The reason? Long ago I learned to discard the hype about energy transfer and knock-down power. Instead, I concentrate scientifically on how the bullet acts between the muzzle and the target, and what it does after it strikes the target.

Bullets in these caliber ranges perform. They perform on the way to the way to the target, and they perform once they get there. They perform because it is possible to combine the right balance of features in a reasonable cartridge, fired by a reasonable rifle. Not surprisingly, then, here at SHWAT we’ve covered the 6.5 Grendel extensively. For example, take a look at our Maker Bullets 6.5 Grendel experience. Remington and Barnes have new  6.5 Grendel options we test while hog hunting. And our very popular comparison of the 6.5 Grendel vs. .308 story has generated a lot of conversation.

As bullet diameters grow larger, getting performance requires much heavier bullets. That, in turn, which requires more powder to move them. Which then creates the need for larger cases and much larger and heavier guns. So when we reach the .308 (7.62mm) or 8mm (.323 cal.) cartridges, we see a compromise toward less effective bullet design in order to keep recoil down, cartridge length at existing standards, and rifles reasonably light.

I have long viewed the 6.5mm cartridge family as ideal for the vast majority of both sporting and military uses. Their trajectories can match larger magnum chamberings and their bullets retain velocities better, allowing reliable expansion and outstanding penetration at ranges beyond what most magnum cartridges allow. The 6.5 mm bullets give us substantially more bullet weight than a .223 or 6mm cartridge.

However, both 6mm and 7mm cartridges are close contenders, and I like and use both, and each shooter must weigh advantages for their particular purpose. Let’s take a deeper dive into the new 6mm ARC.

best 6mm ARC rifle

Lantac is one of the early adopters of the 6mm ARC, and this looks like one impressive rifle!

Hornady’s new 6mm Advanced Rifle Cartridge (ARC) is a 6mm cartridge based on a 6.5 Grendel case. This was a natural evolution, and it is not surprising that Hornady was the one to standardize such a cartridge since they have provided outstanding support for the Grendel from the start. This also makes the 6mm ARC vs. 6.5 Grendel conversation that much more interesting.

Most of the talk I have seen 6mm ARC compares it to the .308. I’ll just say here that in an AR, I’d rather have a 6.5 Grendel or 6mm ARC than a .308.

6mm ARC vs. 6.5 Grendel

The real comparison here is between the 6.5 Grendel and the 6mm ARC.

Both cartridges are compact cartridges. Their case design and capacity are very similar, and both are designed to be fired from an AR-15 (versus a larger framed rifle like an AR-10). This means that neither is going to deliver their best results with heaviest-for-caliber loads, and neither will gain the velocity of longer bolt-action cartridges, such as .234 Winchester or 6.5 Creedmoor.

However, velocity isn’t everything, and pushing a bullet too fast can be detrimental to accuracy. I suspect that Hornady understands this as their 6mm Creedmoor cartridges are only moderately faster than the 6mm ARC.

To keep our 6mm ARC vs. 6.5 Grendel comparison fair, we will compare published ballistics from Hornady. Hornady’s numbers come from 24” test barrels, so velocities may be slightly higher than from some shorter factory barrels. However, I find that Alexander Arms 6.5 Grendel barrels produce consistently higher velocities than other barrels. Hornady ammunition fired from my 18” Alexander Arms barrel averages only 40 fps slower than from Hornady’s 24” test barrel. (It also averages faster speeds than from some 20” and 22” barrels from other manufacturers.)

Also, I don’t know how fast the 6mm ARC builds velocity, so the velocities could be closer between the two cartridges in shorter barrels.

6.5 Grendel vs. 6mm ARC Ballistics

6mm ARC vs. 6.5 Grendel

6.5 Grendel vs. 6mm ARC

As you can see, velocities differ by 170 – 220 fps, with the lighter 6mm ARC moving faster from the compact case. This is exactly what we would expect.

Sectional densities are similar, and both should be good penetrators.

Ballistic coefficients are about equal, with a slight edge going to the 6mm 108 ELD Match. However, even with similar BCs, velocities come into play, and the increased velocity of the 6mm ARC translates to a slightly flatter trajectory.

Downrange velocities show how close the BCs and SDs are between the cartridges. The 175 – 183 fps difference in velocity at 500 yards shows the match cartridges are pretty comparable, with the 6.5 Grendel holding its velocity slightly better with the hunting loads.

Trajectories within reasonable hunting ranges are close. At 500 yards, the difference in drop is about 6 to 7 inches. To me, that difference is negligible.

When it comes to terminal ballistics, the Grendel has proven itself to me, with a lethal effect far in excess of what most people would expect from the wicked little cartridge. It is absolutely devastating on game and penetration is outstanding. I once fired a single 123 gr. Hornady SST completely through two goats, and the intervening space between, penetrating flesh and bone on both goats resulting in impressive exit wounds on both.

Both cartridges are sized for AR-15 rifles or mini-action bolt guns. Both will offer similar ballistics and terminal performance. They increase the lethality and penetration capabilities of the AR-15 well beyond the capabilities of the 5.56mm cartridge. Both vastly increase the versatility of the AR platform, covering varmint hunting, big game hunting, self-defense, and military roles with ease.

6mm ARC bolt action

6mm ARC vs. 6.5 Grendel for Hunting

For hunting varmints, the 6mm ARC would have the advantage. Its lighter bullets are at home here. Given the ideal long-range bullet dimensions, the best long-range 6mm bullet will be around the 112 gr. weight. On the other hand, the best 6.5mm bullets will be closer to 140 gr.  So, the 6mm will be a better choice for launching 95 gr. or 100 gr. varmint bullets, though the 6.5 Grendel is certainly capable.

For hunting deer-sized game, both the 6mm ARC and 6.5 Grendel will work fine. A lot of North American deer have fallen to 6mm and 6.5mm projectiles in the century or more since Winchester, Marlin, and others began offering 6mm chamberings.

6mm ARC

Stepping up to moose, elk, caribou, and black bear – and maybe a wild pig or two – I’d certainly prefer the extra bullet weight of the 6.5 Grendel. Alexander Arms offers a load using the 120 gr. Barnes TSX bullet. It leaves my 18” barrel at 2,526 FPS. This is my go-to cartridge for hunting heavier game with the 6.5 Grendel. I would not hesitate to shoot a moose or black bear with this cartridge and carry it in my gun when hunting lighter game in brown bear country.

And while we’re talking about the Barnes TSX and TTSX bullets… I’m a huge fan. They expand reliably and consistently, yet even with expansion, they usually retain all or almost all their weight and penetrate deeply. They usually exit, even with quartering shots, causing faster bleeding, and easier tracking. I have no qualms about shooting a moose with a Barnes TSX from a 5.56. That will irritate some, but know that I grew up a subsistence hunter here in Alaska and didn’t come to that conclusion as a keyboard commando.

Would I shoot a moose with the 6mm ARC cartridge? Without hesitation. But my first choice for this purpose would be the Alexander Arms load in the 6.5 Grendel. At the ranges at which these bullets will expand reliably, the difference in velocity won’t make a difference, and the extra weight is an advantage.

Which one would I choose, the 6mm ARC or the 6.5 Grendel?

In an AR-15 or a small bolt action, both the 6.5 Grendel and the new 6mm ARC make outstanding big game cartridges for a child, with low recoil even when chambered in the lightest of rifles. A rifle like the CMMG RESOLUTE 6mm ARC rifle, which combines a rifle length gas system with a 16” barrel will soften recoil even further making it an ideal hunting rifle for smaller shooters.

For my own use, I’ll take the 6.5 Grendel over the 6mm ARC, but not by a wide margin. With velocities and trajectories very close, they are both practical for the same purposes. If I were strictly a long-range competition shooter, then I might go with the 6mm ARC as it has some small advantages. But a skilled shooter would do fine with either.

cmmg 6mm arc

Another enticing 6mm ARC option, this one from CMMG

With Hornady behind the 6mm ARC cartridge, it is sure to be popular. I expect we will see companies like Barnes and Norma stepping in with TSX or TTSX loads, or perhaps something of their own design.

So while both are outstanding cartridges, I will stick with the  6.5 Grendel as my all-around, multi-purpose AR cartridge. That said, I would not feel under-gunned at all with the 6mm ARC. I’m also not promising that I won’t ever add one to the collection. CMMG’s 20” ENDEAVOR rifle has me thinking . . .

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