The 6mm ARC, but Why?

I had no intention of getting into another cartridge this year, but then again that’s never stopped me from getting into another cartridge. Regardless, I did. The cartridge is the 6mm Advanced Rifle Cartridge, also known as 6x38mm or 6mm ARC for short. While I’ve only started dabbling with it, 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, but perhaps the more important question is: why?

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

After a few internet searches I learned that the cartridge was introduced by Hornady in 2020 as a low-recoil high-accuracy long-range cartridge designed for the AR-15 platform. Advertised ballistics indicate this intermediate cartridge can send the long and skinny 6mm (.243″) caliber projectiles down range at velocities of 2,800 feet per second out of a 24″ barrel. Long and skinny projectiles, like these, have a generous sectional density (SD) and are often characterized by a high ballistic coefficient (BC). In other words, they shoot flat and can carry quite a bit of energy downrange. These qualities make the 6mm ARC a viable cartridge for hunting medium game (CXP2 class) animals, including cougars and black bears, out to 600 yards or for engaging targets at distances as far as 1,300 yards or thereabouts.

On the surface, the 6mm ARC might not seem that impressive. I mean one can pick up just about any popular rifle hunting cartridge and expect to see similar or further effective ranges for medium game animals. One can also pick up just about any modern long distance rifle cartridge and expect to see similar or further effective ranges. While that’s true, none of those are an intermediate rifle cartridge compatible with the AR-15 platform.

So what? That’s a fair question. I think this cartridge works well in a few situations. For existing AR-15 owners, who want to hunt medium game at longer distances and get into long range shooting it offers a single cartridge that does both of those things well without having to purchase another file for whatever reason. There was a time I was in this position. I remember heavily considering the .224 Valkyrie as an option to start dabbling in long range shooting but was turned off by all of the stability problems folks were having at the time. Last year, I spent some time looking into the 6.5mm Grendel and was actually still considering it when I heard about the 6mm ARC which carries more energy and can maintain supersonic flight for an additional football field or two.

But why bother with the 6mm ARC when one already has a suitable hunting rifle and long range rifle? Also a fair question. The only other reason I could come up with, other than “because you can”, is because sometimes space is a luxury that one doesn’t have. An upper receiver takes up less space than another rifle. While storage space probably isn’t an issue for folks who already have a hunting rifle and a long range rifle today, 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 that I’m enjoying quite a bit. However, for those who are cartridge curious like me there are a few things to be wary of and consider.

First off the 6mm ARC is still new on the scene and there is no guarantee that the market will accept it. Even if the market does accept it, there is no telling how well it will be supported and for how long. Currently, there are a few manufacturers offering complete AR-15s chambered for the cartridge and even more offering complete upper receivers and barrels with lengths ranging from 16″ to 24″. The 6mm ARC utilizes 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 is manufactured by AR-Stoner and I’ve found it to work reliably.

Potential buyers should also be aware there are only three currently available factory ammunition loads available, all of which are manufactured by Hornady. The options are 103gr ELD-X Precision Hunter, 105gr BTHP Black, or 108gr ELD Match.

Even though dies for this cartridge are available from a few manufactures, including Hornady, Lee, and RCBS, hand loading this round also has a few quirks. While there are several 6mm (.243″) caliber projectiles to choose from, the overall length of projectile is limited by the AR-15 magazine that has to fit inside of standard AR-15 magazine well. While I don’t know anything about hand loading, a fellow I respect explained to me that in order to seat the projectile deep enough into the case in order to meet the overall length constraint might mean using less powder or compressing it. Admittedly, I don’t quite understand it. What I do know is that Hornady has supplied two different loading data sheets – one for gas guns and another for bolt guns. It’s worth noting the gas gun data limits pressure to a maximum of 52,000psi and the bolt gun data limits pressure to a maximum of 62,000psi. I can’t say whether the pressure difference has to do with overall length and powder compression, but it is obvious that the pressure limitation places reduces the ballistic potential of the 6mm ARC cartridge for the AR-15 platform.

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 that this post has provided some data points for folks who are considering getting into this cartridge to ponder on. For me? I’m enjoying the cartridge which is proving to be a fun low-recoil high-accuracy cartridge. So much so that one can expect more posts about this cartridge to follow in the near future.