Hornady .338 ARC: The New Standard for Subsonic Ammo?

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The new .338 Advanced Rifle Cartridge (ARC) exists because, like a handful of other Hornady cartridges, the U.S. Department of Defense (DOD) identified a need. The first public ARC cartridge that we learned about was the 6mm ARC, featured in Guns & Ammo’s September 2020 issue. The fact is that .338 ARC pre-­dates the 6mm. In 2017, the DOD wanted more subsonic accuracy and better terminal performance than what was available, so Hornady started development of its .338. However, the DOD quickly moved on to more pressing matters. Several years later, the folks at Defense got back to their unfinished subsonic business, hence the .338 ARC is now public.

Subsonic is Hard to Do Well

Creating a cartridge that performs well at both subsonic and supersonic velocities requires careful analysis of several factors. There’s the bullet to consider, as well as the case volume and powders available to fill it. In a perfect world, a subsonic bullet would have a full case of readily available powder and operate at high pressure. The supersonic version would have a lighter bullet and a different powder that takes up the additional case capacity and also operates at high pressure.

It’s natural to compare the .338 ARC against the .300 Blackout since both fit in AR-­15s and are capable of firing at subsonic and supersonic velocities. The .300 Blackout started out as the .300 Whisper, and both cartridges were built to fire 110-­ to 120-­grain supersonic projectiles in an effort to improve the terminal effects generated by an AR-­15. Good subsonic performance with 220-­grain bullets was also a goal. While the .300 Blackout is a good supersonic cartridge and a decent subsonic cartridge, it doesn’t really excel at either once the target distance gets beyond 100 yards. I’d even go so far as to say that the .300 Blackout and its pedestrian performance is the reason the .338 ARC exists. 

The DOD jumped on the .300 Blackout train around 2010. After six or seven years of working with it, it decided they wanted something else because they weren’t getting the accuracy and terminal effects they wanted at distances exceeding 100 yards. This is why the first ARC cartridge that was developed was this .338. Hornady started working on it by designing a case with the correct volume for subsonic and supersonic use, but subsonic performance got the priority. One of the goals was to keep the chamber pressure of the subsonic load as high as possible without exceeding a velocity of 1,075 feet per second (fps). Keeping chamber pressure relatively high meant the powder would burn evenly. This kept muzzle velocity consistent with low extreme spreads and prevented fouling the chamber and receiver with unburned powder, a common occurrence when shooting subsonic .300 Blackout. The high (for subsonic) chamber pressure is one of the secrets to its tight velocity standard deviation and is why accuracy at 200 and 300 yards is much better than the .300 Blackout, meeting the DOD’s needs.

The bullet weight for the .338 ARC in subsonic form tips the scales at 307 grains. Hornady’s Sub-­X bullet has a flat meplat featuring a large rubber plug filling the cavity in the bullet’s nose. This bullet was designed for great accuracy and excellent terminal effects at subsonic velocity with projectiles typically penetrating 16 to 20 inches of ballistic gelatin. During testing, Hornady’s Sub-­X load for .338 ARC exhibited the tight velocity extreme spread and standard deviation, with an extreme spread of 42 fps and a standard deviation of 15.5 fps. This is a little better than average for quality factory supersonic ammunition, but is exceptional for any subsonic ammunition.

The reason velocity is such a performance issue for subsonic cartridges is the effect it has on the bullet’s time of flight, especially as the distance to the target increases. As soon as the bullet leaves the muzzle, gravity pulls it toward the ground. The projectile’s downward velocity starts at zero and accelerates at 32.2 fps. A velocity extreme spread of 40 fps results in 1.3 inches of drop at 200 yards and 13.2 inches of vertical dispersion at 300 yards. No single variable impacts group size at 200 or 300 yards with a subsonic cartridge like velocity variation.

But It's Super, Too!

The .338 ARC isn’t just a subsonic cartridge; Hornady also loads a 175-­grain supersonic hollowpoint bullet in its Black line. The Match projectile has a flat bottom, thin copper jacket and massive open cavity inside the nose. Out of a 16-­inch barrel, it has a muzzle velocity of 2,084 fps and generates 1,000 foot-­pounds (ft.-lbs.) of energy out to 250 yards. However, the impact velocity of this bullet only stays above 1,800 fps out to 150 yards. I wouldn’t plan on any expansion or fragmentation beyond that distance.

The 175-­grain hollowpoint load is a great choice for ringing steel and would work well for hog hunting inside 150 yards. I don’t think I’d hunt deer with it because there’s a decent chance it would just pass through the animal with minimal upset. I’m also pretty sure that Hornady will come up with a .338 ARC load ideal for hunting with a bullet designed to expand at ARC velocity. This is a task well within Hornady’s skillset.

When developing the supersonic load, Hornady kept an eye on chamber pressure. All of the ARC cartridges are capped at a maximum chamber pressure of 52,000 pounds per square inch (psi) because they are designed to be functional in AR-­15-type rifles. Handloaders can safely take the pressure up to 62,000 psi when developing loads for bolt-­action rifles, which would add about another 150 fps.

The benefit of Hornady’s attention to pressure is that the subsonic load generates about 36,000 psi chamber pressure. Keeping the chamber pressure as high as possible for the subsonic load means an AR-­15 can reliably cycle supersonic and subsonic loads without having to adjust the gas block or swap out buffer weights. The .300 Blackout could only pull this off with a pistol-­length gas system and a large gas port, which meant it was way over-gassed when shooting supersonic if it cycled subsonic loads reliably. This is because supersonic .300 Blackout loads have a maximum chamber pressure of 55,000 psi and the subsonic loads operate at around 25,000 psi.

Why I Like It:

I’ve been on the lookout for a good subsonic cartridge that wasn’t useless in supersonic form for years. The .300 Blackout is a good supersonic cartridge, but only OK in subsonic form. It also struggles to headspace properly with factory ammunition in bolt-­action rifles. I saw a much higher frequency of rifles going click when they should have gone bang. This occurs because the .300 Blackout has an unimpressive shoulder that leaves no room for case shape error. I am mostly interested in bolt-­action subsonic rifles because I want to be able to shoot as quietly as possible while still generating the best terminal effects. I also want the option for strong supersonic performance on whitetail and hogs.

The 8.6 Blackout came along and I got my hopes up. It has plenty of powder capacity for fantastic supersonic performance. Subsonic accuracy might struggle with the velocity standard deviation because of that same case capacity, but I thought I’d take a look. I even went so far as to take the 8.6 Blackout on a subsonic deer hunt and got a nice little buck in the process. That’s where the good news ended. The twist rate for the 8.6 Blackout is 1-in-3 inches. This means that only monolithic bullets will work at supersonic velocity, and only monolithic bullets will expand reliably for good terminal effects at subsonic velocity. I’m not a huge fan of monolithic bullets because they are expensive and they don’t group well at subsonic (and often supersonic) velocity. Monolithic bullets are hard and don’t obturate to engage the bore’s rifling like lead-­core bullets. The 1-in-3-­inch twist rate of the 8.6 Blackout causes a lot more problems than it solves.

I did all of my testing of the .338 ARC with a Proof Research Conviction, a fine bolt-­action rifle. The .338 ARC was designed for the AR-­15 (just like the 6mm ARC and .22 ARC), but the ARC family is a great choice for bolt-­action rifles, too. They offer low recoil, are easier to shoot accurately, and nothing is quieter than a bolt-­action when shooting subsonic and suppressed. Proof Research uses bolt faces designed just for the ARC cartridges, so extraction and ejection are positive. Accuracy is always top-­notch.

The .338 ARC answers the mail on everything I’m looking for in a subsonic cartridge. It offers the most consistent velocity spreads of any subsonic cartridge I’ve tested, and it works well with lead-core bullets. In supersonic form, it lands between the .350 Legend and .400 Legend for the energy it creates. Once a good hunting bullet comes along, it’ll do everything supersonic I want. The final piece is it’s highly likely to see DOD adoption. The 6mm ARC is alive and well and gaining popularity fast inside DOD. That means rifles designed around the ARC family are in use with U.S. Special Operations. All they need for great subsonic performance is a new barrel. The .338 ARC will never be as popular as the 6mm ARC, but it has good reasons to thrive in the next few years. If you want to shoot an accurate and lethal subsonic or supersonic rifle cartridge, try the .338 ARC. It rates top billing. 

Proof Research Conviction Specifications

• Type: Bolt ­action
• Cartridge: .338 ARC (tested)
• Capacity: 10+1 rds.
• Barrel: 16 in., 1:8-­in. twist
• Length: 36.25 in.
• Weight: 7 lbs., 10 oz.
• Stock: Proof Research
• Grip: Vertical
• Length of Pull: 13.6 in.
• Finish: Nitride (steel)
• Trigger: TriggerTech Special, 1lb. to 3.5 lbs., adjustable
• Sights: None
• MSRP: $8,500
• Manufacturer: Proof Research, 406-­756-­9290, proofresearch.com

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