.338 ARC (Advanced Rifle Cartridge) is Hornady's AR-15-compatible big-bore cartridge, built by necking up the 6mm ARC case to accept a .338-diameter bullet. Both 6mm ARC and .338 ARC share the 6.5 Grendel case head, which means they use the same Type 2 bolt face and the same magazines. The cartridge fits inside a standard AR-15 magazine well, runs in a standard AR-15 lower, and only requires a new barrel and a Grendel-pattern bolt to convert an existing 5.56 upper.

Hornady's design goal was a single AR-15 cartridge that cycles reliably both supersonic and subsonic, with enough bullet weight to hit harder than 300 Blackout in the role where 300 BLK has dominated for a decade: short-barreled, suppressed rifles for close-range work on game and predators. The .338 caliber is significant. .30 Caliber (.308) is the ceiling for traditional AR-15 suppressed subsonic cartridges, and a .338 bullet has roughly 20% more frontal area at the same length. Combined with heavier bullet weights up to 307 grains, that translates directly into more energy on target.

Hornady currently sells two factory loads, and each does a different job. The 175 gr Black HP Match supersonic is the hunting and predator load. The 307 gr Sub-X subsonic is the suppressed-carbine load. Treat them as two cartridges in one chamber, because the use cases barely overlap.

With the 175 gr Black load zeroed at 200 yards, point of impact is 4.2 inches high at 100 and 16.3 inches low at 300 yards. That is a usable AR-15 carbine trajectory inside 200, comparable to 300 Blackout supersonic but with more retained energy thanks to the heavier projectile. Past 300, drop opens up quickly because the BC is mediocre by modern intermediate standards. If you want to ring steel past 300 yards from an AR-15 lower, see our 5.56 ammo selection guide for Mk262-class loads that hold velocity better at distance.

The 307 gr Sub-X is the more interesting half of the system. At 1,050 fps it carries roughly 750 ft-lbs of muzzle energy, about 1.5 times what a 220 gr 300 BLK subsonic produces. Combined with the larger .338 frontal area, that delivers meaningfully harder hits on close-range game when shot through a suppressor. The trade is range. Past 100 yards a 307 gr bullet at 1,050 fps drops fast and bleeds energy quickly, so treat it as a 100-yard cartridge and zero accordingly.

.300 Blackout is the cartridge .338 ARC is trying to displace, and the comparison is the question most prospective buyers actually want answered. The honest answer: .338 ARC hits harder, especially subsonic, and .300 Blackout is cheaper, more available, and has a deeper ecosystem. Pick based on use case, not on which is “better.”

If you already own a 300 Blackout setup, .338 ARC is a lateral move at best. The energy gain is real, but the cost of switching (new bolt, new barrel, new magazines, new ammo supply) eats the difference. For a deep look at where 300 BLK fits and what it does well, see our 300 Blackout guide and best 300 Blackout ammo ranking.

If you are building from scratch and the use case is suppressed close-range hunting or pig work, .338 ARC has a real argument: the subsonic hit is noticeably harder and expansion is more reliable. For everything else (range practice, home defense, plinking, any kind of supersonic precision work), 300 BLK or 5.56 is a better answer.

These three cartridges share a vague “AR-15 alternative caliber” identity but solve different problems. 6mm ARC is a distance cartridge: it pushes a high-BC 108 gr bullet to 2,750 fps and holds supersonic past 1,000 yards. .350 Legend is a straight-walled hunting cartridge designed for straight-wall-only deer states (Ohio, Iowa, Indiana, Michigan) and runs a 150 gr bullet to 2,325 fps. .338 ARC sits between them in a different niche entirely: short-range, big-bore, subsonic-first.

If you want one AR-15 to ring steel past 600 yards, 6mm ARC. If you hunt deer in a straight-wall state, .350 Legend. If you want maximum energy on hogs at suppressed close range, .338 ARC. There is no overlap worth arguing about.

The .338 ARC upper market filled in faster than most first-year cartridges. Hornady seeded the ecosystem by partnering with multiple AR builders before launch, and the result is a real menu of options across barrel lengths and price points.

• Palmetto State Armory: Complete uppers in 7.5", 8.5", 12.5", and 16" barrel lengths. The value-tier entry point.
• Faxon Firearms: 12.5" match-grade suppressor-ready barreled upper, a step up in barrel and gas system quality.
• KAK Industry: 8.5" complete upper with M-LOK handguard, value tier.
• Pro2A Tactical: 7.5" and 12.5" complete uppers and AR pistol kits.
• Grendel Hunter: Custom barreled uppers with build-your-own configuration, aimed at handloaders and bolt-action conversions.
• Savage Arms: Model 110 bolt-action rifle chambered in .338 ARC for shooters who want maximum velocity out of a longer barrel.
• CMMG: Published guide content and ammo support; check current stock before planning a build around a CMMG upper.

To convert an existing AR-15 upper, you need a .338 ARC barrel and a Grendel-pattern bolt (the same Type 2 bolt face used by 6.5 Grendel and 6mm ARC). You can reuse your carrier, gas system, and lower. Magazines: 6.5 Grendel and 6mm ARC AR-15 magazines (E-Lander, Duramag, ASC) feed .338 ARC, typically at 25 rounds in a 30-round-body magazine. If you want to plan a .338 ARC carbine alongside other AR builds, our rifle builder and comparison tool cover the components that carry over.

.338 ARC needs a suppressor with at least a .338 bore. Most dedicated 5.56 and .30-caliber cans (.224 and .308 bore) will not pass a .338-diameter bullet without a baffle strike. The practical options are multi-cal cans rated for .338 Lapua Magnum or larger: SilencerCo Hybrid 46, Dead Air Nomad-XC, Q Trash Panda, KAC AAC 762-SDN-6 with .338 conversion, and similar large-bore designs.

The upside of a .338-bore can: it covers everything from 5.56 up through .338 Lapua in one footprint, which is part of why the multi-cal can market has exploded since the One Big Beautiful Bill Act zeroed the federal NFA tax on suppressors in January 2026. For the full breakdown of mounts, gas, and tuning, see our AR-15 suppressor setup guide and adjustable gas block guide for cycling a .338 ARC carbine reliably across subsonic and supersonic loads.

.338 ARC makes sense for a specific buyer profile, not everyone. The cartridge solves a real problem for these shooters:

• Suppressed hog and predator hunters who want more energy on target than 300 BLK subsonic delivers at close range. The 307 gr Sub-X is the strongest argument for the cartridge.
• Big-bore AR-15 enthusiasts building a dedicated short-barrel suppressed carbine and willing to absorb ammo cost and Hornady-only supply for the ballistic gain.
• Shooters in supersonic-restricted areas where subsonic loads are required for noise ordinances or hunting regulations and a harder hit matters.
• Existing 6.5 Grendel or 6mm ARC owners who can reuse their bolt and magazines, dropping the conversion cost to just a new barrel.

The cartridge is wrong for most AR-15 buyers, and saying so up front saves money. Skip .338 ARC if you are:

• Already invested in 300 Blackout. The energy gain does not justify replacing a working suppressed setup with a more expensive, supply-limited alternative.
• Looking for cheap practice ammo. There is no .338 ARC range load and there will not be one soon. Plan on $1.50-1.75 per round, no exceptions.
• Shooting past 200 yards. 6mm ARC, 6.5 Grendel, or even quality 5.56 with heavy projectiles will out-shoot .338 ARC at any distance where ballistic coefficient matters.
• Ammo-availability-conscious. Hornady is the only factory loader. One supply disruption, one priority shift, and the supply pipeline narrows.
• A handloader on a budget. Brass is scarce, bullet selection in .338 is geared toward .338 Lapua and .338 Win Mag, and most projectiles are too long for AR-15 magazine length.

.338 ARC is a narrow-purpose cartridge that does its narrow purpose well. For a suppressed AR-15 built around close-range hog and predator work, the 307 gr Sub-X load is the hardest hit you can get out of an AR-15 magazine well, and the ecosystem of uppers from PSA, Faxon, KAK, and Pro2A means you can buy in at any budget. The 175 gr Black supersonic is the weaker half of the system, usable but unremarkable next to 300 BLK supersonic and well behind 6 ARC at any distance.

The two real risks: ammo supply concentrated in one manufacturer (Hornady), and price per round that will not drop until a second loader enters the market. If both of those move in the right direction over the next 12-18 months, .338 ARC has a real shot at being the new default for suppressed AR-15 builds. Until then it is a deliberate choice, not a default. Build with eyes open about the cost.

Cross-reference: for a broader look at AR-15 caliber options and tradeoffs, our 300 Blackout guide covers the cartridge .338 ARC is competing against, the 5.56 ammo selection guide covers what makes a usable supersonic AR-15 round, and the 5.56 suppressors guide is a good starting point on suppressor selection criteria that carry over to .338-bore cans.