Building A 6mm ARC Upper

Making the most out of a long-range round requires top-shelf components.

posted on December 12, 2023
6mm ARC upper.

Over this past summer, I had the opportunity to join Leupold Optics and Hornady Ammunition out in Oregon at Leupold Academy, an intensive study into what makes optics and ammunition tick. We had classroom time learning about the various components that go into optics, followed by ballistic overload with Hornady. It was informative, there’s absolutely no question there, and one of the things that really stood out to me was the long-range performance of Hornady’s new 6 mm ARC cartridge.

We had two full days out on the range working with Seekins Precision rifles. On the first day, we took bolt-action Havak rifles in 6.5 Creedmoor out to 1,500 yards, and on the second day we took semi-automatic DMR rifles in 6 mm ARC out to 1,000 yards. This was the first time I’ve ever connected at 1,000 yards with a semi-automatic rifle, and the ease with which this was accomplished with the 6 mm ARC made it an imperative to add one to the armory. However, while the Seekins rifles were absolutely magnificent on the range, both accurate and reliable, I enjoy building my own, so I gathered tools and components and got to work.

Rather than build a complete rifle, I opted to add a new upper in 6 mm ARC to the armory. Looking through the safe for a suitable lower, I came across the perfect one from a build I put together a few years ago. Since the lower receiver was from Aero Precision, I opted for a complete upper receiver and Atlas S-One handguard from Aero, both in FDE to match the lower (hey, we gotta coordinate, right?). The lower has an excellent Rise Armament RA-434 single-stage trigger in it for accurate shots, and combined with Magpul’s UBR adjustable stock, should make for a rock-solid, stable platform indeed.

With the upper and handguard chosen, it was time for one of the three most critical components in an AR build: the barrel. I have been a fan of Proof Research’s barrels for a very long time, starting nearly a decade ago when I attended a writer’s event where an AR-10 in .308 Win. was in use for the better part of an entire day, firing hundreds of rounds, and the Proof Research 20-inch barrel never heated up or opened up groups. We’ve featured rifles with Proof Research barrels in the pages of Shooting Illustrated, and every one is an excellent shooter. An 18-inch barrel in 6 mm ARC and a gas tube were added to the component list.

The second critical component in an AR build is the bolt-carrier group (the third component is, of course, the trigger), and for that I turned to Faxon Firearms. I’ve included a number of Faxon components in both rifle and pistol builds and have been quite impressed with the performance of everything from the company. Faxon also sent an ambidextrous charging handle with the BCG, leaving only the gas block and muzzle device to complete the upper. Wilson Combat’s Lo-Profile gas block and Q-Comp muzzle brake rounded out the components for the 6 mm ARC build.

Gas block holder
The XS Sights upper receiver block and Wojtek Weaponry gas-block tray proved to be essential for this build.

Building an AR-15 upper receiver, especially with the forward assist and ejection-port door installed from the factory, is a pretty straightforward task. There are a few tools that make it even easier: the XS Sights Armorer’s block helps anchor the upper receiver so the barrel nut can be properly torqued into place (and the muzzle device, too), while a little piece of 3D-printed polymer gas-block tray from Wojtek Weaponry makes installing the gas tube roll pin a much simpler task. Seriously. At $17.50, even if you are only going to build one AR-15 upper receiver in your life, it is well worth it. Getting the gas tube and gas block to line up while holding a roll-pin punch and roll pin and gently tapping with a small hammer is hard enough without the gas block and gas tube squirming all over the place. With the gas block tray from Wojtek, it’s quite a bit easier…

The final piece on the upper had nothing to do with the build, but will come in quite handy on the range. Leupold’s excellent Mark 5HD line of scopes have been fantastic performers, and while out in Oregon we had the excellent 2-10X models on our 6 mm ARC rifles. I found that my eyesight could have used a bit more magnification at the farther ranges, so I opted for the 3.6-18X Mark 5HD scope for this project. The Mark 5HD is secured to the Aero upper with Leupold’s Mark AR IMS (Integral Mounting System). Leupold understands the importance of a solid mount for your optic, and has both individual ring options (more popular for bolt-action rifles) and the one-piece IMS mounts common to AR-style rifles.

This concludes the build portion of the 6 mm ARC AR-15 upper, and makes for a handy stopping point (okay, okay, I’m waiting for it to get a bit warmer and a little less windy to test the rifle). The upper has passed all safety checks (headspace is fine, rounds load and eject without incident), next up is zeroing the optic and then testing the rifle at our protocol distance of 100 yards. If all goes well, we’ll be able to ring some steel at farther distances, too.


Sheriff Jim Wilson
Sheriff Jim Wilson

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