The Hornady 6mm ARC for the Desert Tech MDRX

It seems all too frequent nowadays for a new cartridge to jump to the front of every blog, magazine, and ad campaign.
Ammunition manufacturers are always looking for the next best thing to sell. I cant blame them, and I’d much prefer they spend all the money on R&D so the rest of us don’t have to.
At the top of the ammunition game is the big red H that we have all come to know quite well. Hornady has brought some extremely popular cartridges to market in the recent past, the PRC family comes to mind, as does the revered 6.5 Creedmoor.
This year Hornady has again brought another impressive project to the shooting public, or at least legitimized an older one. The 6mm Advanced Rifle Cartridge (ARC) is that new product, and it looks to become as popular as it’s other red-tipped siblings.
The 6mm ARC is essentially a 6mm Grendel, but legitimized by Hornady’s production. It shares a few basic dimensions with the Grendel, but necked down to .243/6mm. It shoots heavy for caliber bullets in the 90-110 grain range, from a 7.5 twist barrel.
Hornady currently offers three different loads for the ARC, a 103 grain ELD-X in the Precision Hunter line, a 105 BTHP in the Black ammunition line, and a 108 grain ELD Match in their match ammunition line. In addition to these different ammunition lines, Hornady has also released loading dies, and components for loading the ARC.

Having seen many of these new cartridges come and go, I was cautiously optimistic for several reasons.
Just because it’s new doesn’t alway make it better, but I had long been considering a 6mm small frame AR cartridge like the Grendel or the 6mm Rat. The slightly larger bore of the 6mm gives a significant advantage over .224 caliber bullets, and if the velocity is there then you don’t have much to lose.

For those who dont already know, the MDRX is a multi-caliber ambidextrous bullpup rifle. Its closest peers are rifles like the Steyr AUG, the IWI X95 or T7, but the MDRX brings to the table a few more advantages. The MDRX will allow the use of both large and small frame cartridges, something the others will not do. Not only can the MDRX change between an assortment of calibers, but it can also be swapped in minutes with a single hex-key wrench. At the time of writing this, I have seven different barrels or conversion kits for my MDRX, they vary incredibly in their purpose and use. For cheap plinking, there is the traditional 223 (available in 16 or 20 inch barrels), for subsonic shooting there is the 16 inch 300Blk, for heavy thumping you can run the 308 Win (16 or 20 inch), and for distant shooting, you can run the 20 inch 6.5 Creedmoor. These four are available from the factory, I also have several custom barrels for my MDRX; the 450 Bushmaster brings devastating power to this tiny rifle. The 350 Legend is another that fits more of a niche hunting purpose, and today’s subject, the 6mm ARC is my latest addition to the collection. The 6 ARC brings inexpensive accuracy to the MDRX, it’s almost like a hybrid of my 223 barrel and 6.5 Creedmoor. It is inexpensive to load and shoot, has very negligible recoil, but shoots like a 6.5 Creedmoor as far as drop, and wind deflection. And the fact that it shoots so accurately makes this conversion kit perhaps my most favorite in the group, it rivals my SRS A1 as far as accuracy is concerned. Watch the video at the bottom of this article

So why the 6MM ARC?
Small frame autoloading rifles already have untold options when it comes to caliber, so what makes the ARC different? According to Hornady there are several reasons. The first one I’ll mention is performance, the ARC produces a similar if not superior ballistic curve than 308 Winchester. It maintains velocity and drop further than the 308 Win. It does this while showing off the second reason, efficiency. The ARC uses smaller, lighter cartridges with lighter powder charges to obtain this superior ballistic advantage. It also maintains a higher level of energy on target than it’s small frame competitors, the hundred-plus grain bullets carry a better energy load than your typical 69 to 90-grain bullets fired from AR’s. The overall load carried by a shooter or soldier is also less because of the ARC’s smaller size and weight when compared to larger cartridges like the 308. This reduction in weight, and powder charge also reduces the recoil felt by the shooter. This allows for rapid hit/miss confirmation and quick follow-up shots.

Of course, all this only matters if the ARC can shoot accurately. For me only accurate rifles are interesting, so I was happy to see how the ARC performed both on paper up close, and out at distance. The very first time I shot the 6 ARC at an actual range, I put three shots onto an IPSC target at 200 yards without even zeroing my scope.

After a little bit of research and some thought, I decided I would have an ARC of my own. Hornady shows a litany of manufacturers on their website that chamber rifles in the 6mm ARC, but I wanted to try something else.
My Desert Tech MDRX is a perfect candidate for a cartridge like the ARC, it is a multi-caliber platform that is easily adapted to large or small frame cartridges.
My good friend and talented gunsmith Eric at ES-Tactical got ahold of a quality 6mm barrel blank, and we set to working. The twenty-inch 7.5 twist barrel came from K&P, it was drilled, chambered, fluted, and threaded. With the appropriate barrel extension and gas block installed, all I needed was a bolt. The ARC uses a slightly larger bolt face than the 223, which took a little steady machining but worked perfectly. I also had to machine a little bit off of the 308 ejection chute clip in order to get it to firmly hold the smaller 6ARC cases.

The ARC runs at similar pressures to its peers, so I used the same gas settings as a 308 Winchester. And in a matter of a few minutes, the 6mm ARC roared to life.

Range trip
As I mentioned above, the first actual range trip for the ARC was impressive. The MDRX had been zeroed for my 223 barrel, but the POI was very close for the ARC. So close in fact, I shot 200, 450, and 550 yards without even zeroing the scope. Shooting standard size IPSC steel targets is not record breaking accuracy or anything, but it felt good right out of the gate.
Its fairly well known that the Grendel doesn’t like to feed well and doesn’t from 556 magazines, and the ARC shares that family trait. But I temporarily tried some P-mags until I got the proper magazines.  The 300BLK magazines worked better than 556 mags, but only if you loaded a few. I used a couple different magazines from Dura-Mag to avoid feeding issues. With these purpose-built magazines, you can load them up full, and have flawless function, like any other AR-type magazine.

ten and twenty round 6.5 Grendel magazines from Dura Mag worked flawlessly with the ARC

Recoil on the ARC is as Hornady suggested, minimal. Seeing your own hits on steel targets is easy at medium range, and even easier at long range.

A good five-shot group from the 6ARC at 100 yards

The accuracy of the ARC is superb, very likely due to the quality barrel and machining. But no doubt that the cartridges design also aids in keeping my groups together. Both factory Hornady match ammunition as well as my handloads performed well, producing groups that averaged around .5-.75 MOA and some of the best groups have been in the .3 to .4 MOA.
I was very impressed, it seemed to be the most consistent shooting barrel I have for this rifle. I was using my US Optics TS8X, which is significantly less magnification than I typically use when shooting groups. The RBR reticle is calibrated for 5.56 ammunition, but I figured it would be close with the 6 ARC. After shooting a few five-shot groups, I reached out to 300 yards across a canyon on a rock that was about ten inches wide. After hitting it over and over, I figured it was worth trying something further, but the only other target-sized rock I could find was at 960 yards. Not having a drop chart made yet, I did a little guestimating on my holdover. I was close, but shot over it with an 8 MRAD hold. So I dropped to 7 MRAD, and made a better windcall, and sent a second shot, which found my point of aim with nearly perfect precision. I was more than dazzled, as I continued to place shots on targets all over the mountain.

Loading the ARC
I’ve been handloading for many years, so loading the 6mm ARC was as simple as switching out some dies. The powder charges were pleasantly light, I used both CFE556 and BLC-2 for the ARC. Both performed well and provided good accuracy and consistency over CCI BR4 primers. I followed the load data that Hornady has available on their website, around 28 grains of powder was where I settled. The Hornady 105 grain BTHP was the bulk of my loading fodder, it is not too expensive, and performs very well. We have since used the ARC in a deadly encounter with some Wyoming antelope.

I’m usually slow to jump on new trends, it took me some time to pickup even the 6.5 Creedmoor. But this little cartridge has definitely piqued my interest, so much in fact I haven’t used any other barrels in my MDR since I got this one. Its accurate, smooth shooting, easy to spot hits and misses. And it hits targets pretty hard even at some significant distances, the only drawback I can even come up with is that I need to keep close tabs on the brass. Its a little more hard to come by than the average case, so I gotta keep an eye on them.

This awesome little cartridge is staying right close to me, we’ll be taking it hunting this fall for sure. And I don’t think it will be going away soon.
New things aren’t always better, but in the case of the 6MM ARC, I think Hornady has hit a 10X.


Watch the viedo to see the 6 ARC MDRX in action

3 thoughts on “The Hornady 6mm ARC for the Desert Tech MDRX

  1. I have been eyeing the 6mm ARC as a potential for my next caliber conversion. If this was available for either the MDR or the XCR-L, I think I would already have one. The question is who will get there first.


    1. Well, I’d assume you’d have to make one yourself as I did. Not many people making them right now because parts are so hard to find. I assume it could be done on an X95, but I sure wouldn’t. Not when the MDRX is such an easy platform to do it on.


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