The 6mm ARC, but Why?

I had no intention of getting into another cartridge this year. Then again, that’s never stopped me before. Regardless, I did. The cartridge is the 6mm Advanced Rifle Cartridge, also known as 6x38mm or 6mm ARC for short. I’ve only started dabbling with it. Even so, I’m getting a lot of questions about it, such as: what is the 6mm ARC? I’ll cover that to the best of my ability in this post. Perhaps the more important question, though, is: why?

Let me back up to the second quarter of 2021. That’s when the buzz around this cartridge reached me, from some folks that I think highly of. Initially, I figured this was yet another trendy boutique cartridge for the AR-15 platform. It wasn’t worth paying attention to, so I dismissed it. However, the buzz remained strong, and I finally decided to dig into it.

What Is the 6mm ARC?

After a few internet searches, I learned that Hornady introduced the cartridge in 2020. It’s a low-recoil, high-accuracy, long-range cartridge for the AR-15 platform. Hornady’s advertised ballistics put long, skinny 6mm (.243 in) projectiles at 2,800 fps out of a 24 in barrel. Long and skinny projectiles like these have a generous sectional density (SD) and often carry a high ballistic coefficient (BC). In other words, they shoot flat and carry quite a bit of energy downrange. Consequently, the 6mm ARC is viable for medium game (CXP2 class), cougars and black bears included, out to 600 yd. It can also engage targets at distances as far as 1,300 yd or thereabouts.

Hornady’s 11th Edition Handbook of Cartridge Reloading, which I didn’t have when I first wrote this, tells the design story. The case comes from the 6.5mm Grendel, with the shoulder and neck reworked for long, sleek 6mm bullets. The reason is the AR-15 magazine. At a 2.260 in overall length, the Grendel tops out at 123 to 125 gr bullets. The sleek 130 to 153 gr 6.5mm bullets are simply too long to seat. So Hornady built the 6mm ARC to fit and stabilize long 108 to 110 gr 6mm bullets at that length. The standard 1:7.5 twist handles the heaviest 6mm bullets in Hornady’s catalog.

On the surface, the 6mm ARC might not seem that impressive. One can pick up just about any popular rifle hunting cartridge and expect similar or longer reach on medium game. The same goes for just about any modern long-distance rifle cartridge on targets. While that’s true, none of those are an intermediate rifle cartridge compatible with the AR-15 platform.

Why the 6mm ARC?

So what? That’s a fair question. I think this cartridge works well in a few situations. Take an AR-15 owner who wants to hunt medium game at longer distances and get into long-range shooting. The 6mm ARC offers that shooter a single cartridge that does both well. There’s no need to purchase another rifle for either one. There was a time I was in this position. I remember heavily considering the .224 Valkyrie as a way to start dabbling in long-range shooting. All of the stability problems folks were having at the time turned me off. Last year, I spent some time looking into the 6.5mm Grendel. I was still considering it when I heard about the 6mm ARC. The 6mm ARC carries more energy and stays supersonic for an additional football field or two.

But why bother with the 6mm ARC when one already has a suitable hunting rifle and a long-range rifle? Also a fair question. The only other reason I could come up with, other than “because you can,” is space. Sometimes space is a luxury one doesn’t have. An upper receiver takes up less space than another rifle. Storage space probably isn’t an issue today for folks who already own a hunting rifle and a long-range rifle. That doesn’t mean it will never be an issue, and it’s nice to have options.

It should be obvious that I’m not advocating for everyone to go get into the 6mm ARC right away. I think it’s an impressive cartridge, and I’m enjoying it quite a bit. However, for those who are cartridge curious like me, there are a few things to be wary of and consider.

Before You Buy Into the 6mm ARC

Market Support

First off, as I write this in late 2021, the 6mm ARC is still new on the scene. There is no guarantee that the market will accept it. Even if it does, there is no telling how well the cartridge will be supported and for how long. Currently, a few manufacturers offer complete AR-15s chambered for the cartridge. Even more offer complete upper receivers and barrels, with lengths ranging from 16 in to 24 in. The 6mm ARC uses the same bolt head size as the 6.5mm Grendel, which seems to have healthy market support. Magazines are another story. While some folks have used 6.5mm Grendel-compatible magazines successfully with the 6mm ARC, others have reported problems with them. As of writing, the only 6mm ARC-specific magazine I have found comes from AR-Stoner. It has worked reliably for me.

Factory Ammunition

Potential buyers should also be aware that, in late 2021, there are only three factory ammunition loads. Hornady makes all of them. The options are 103 gr ELD-X Precision Hunter, 105 gr BTHP Black or 108 gr ELD Match.

Hand Loading the 6mm ARC

Dies for this cartridge are available from a few manufacturers, including Hornady, Lee and RCBS. Even so, hand loading this round has a few quirks. There are several 6mm (.243 in) projectiles to choose from. However, the AR-15 magazine caps a loaded round at 2.260 in overall, so the longest bullets must seat deep. I don’t know anything about hand loading, but a fellow I respect explained it to me this way. Seating a long projectile deep enough to meet that limit might mean using less powder or compressing it. Admittedly, I don’t quite understand it.

Hornady, for its part, publishes two sets of load data: one for gas guns and one for bolt guns. The gas gun data limits pressure to 52,000 psi. Bolt gun data allows 62,000 psi. Hornady’s handbook explains the difference, and it isn’t about powder compression. The gas gun limit protects the AR-15 bolt, which is the weak link in the platform. Bolt gun data runs above the SAAMI specification and applies only to bolt-action rifles built for it. Do not run bolt gun loads in a gas gun. Either way, the pressure limit reduces the ballistic potential of the 6mm ARC in the AR-15 platform. In a bolt gun with a 24 in barrel, Hornady’s data launches a 110 gr A-Tip Match at 2,850 fps. Each extra inch of barrel adds about 30 fps.

Closing Thoughts

So there you have it. Everything I know about the 6mm ARC at the moment. Is the 6mm ARC for everyone? No. Is it for you? I don’t know. Perhaps it is. Perhaps it isn’t. That’s for you to decide. However, I hope this post has provided some data points to ponder for folks considering the cartridge. For me? I’m enjoying the cartridge, which is proving to be a fun, low-recoil, high-accuracy round. Before this post, I had already compared it on paper with the .220 Swift and .25-06 Remington for deer. Since then, I’ve written up the Aero Precision upper and M4E1 lower I built for it. One can expect more posts about this cartridge to follow.