.338 ARC vs 8.6 Blackout: Which Big-Bore Fits Your Build?

If you’re stepping up from 5.56 or 6.5 and want a hard-hitting, compact rifle that carries serious energy downrange, two modern cartridges lead the charge the .338 ARC and the 8.6 Blackout. At first glance, both promise “big-bore” performance in shorter rifles, but their engineering, purpose, and platforms couldn’t be more different.

In this Redleg breakdown, we’ll cover:

✅ Same bullet performance comparison

✅ Optimal bullet weights for each caliber

✅ Platform pros and cons (AR-15 vs. AR-10 vs. bolt)

TL;DR (Too Long; Didn't Read) .338 ARC vs 8.6 Blackout Summary

• .338 ARC → AR-15 platform, SAAMI-standardized, versatile for both supersonic and subsonic use, and easy to build with Grendel-type bolts and magazines.

• 8.6 Blackout → AR-10 or bolt platform, built from the 6.5 Creedmoor case, designed for short barrels and suppressor use, especially with heavy subsonic bullets and ultra-fast twists.

If you want precision and versatility in a lighter rifle, the .338 ARC wins. If you want suppressed, devastating short-barrel performance, the 8.6 Blackout is unmatched.

Same Bullet Comparison (Apples to Apples)

To keep things fair, let’s compare both cartridges using the same 175-grain bullet a class they can both stabilize.

• .338 ARC: Factory loads run around 2,075 fps from a 16–18" barrel, delivering about 1,670 ft-lbs of energy.

• 8.6 Blackout: When loaded supersonic with the same bullet in a tuned 12–14" barrel, velocities hover near 2,150–2,250 fps, hitting roughly 1,850 ft-lbs.

The 8.6 Blackout has a small energy advantage when optimized, but the .338 ARC has consistency predictable chamber specs, pressure, and factory ammo backed by SAAMI. In short: ARC = consistency and ecosystem, 8.6 = raw thump when tuned.

Optimal Bullet for Each Caliber

Subsonic and Suppressed

.338 ARC:

Runs beautifully with 300–307-grain subsonic bullets, like the Hornady Sub-X. Designed to expand at ~1,050 fps, they’re excellent for close-range hunting and suppressed applications all within an AR-15 footprint.

8.6 Blackout:

Built for heavy subs 300–340-grain projectiles stabilized by extreme 1:3 to 1:4 twist barrels. The fast spin increases yaw and fragmentation, delivering devastating terminal performance at close to mid ranges.

Winner: 8.6 Blackout, if your priority is suppressed hunting or short-barrel power.

Supersonic Hunting and General Use

.338 ARC:

Best with 160–200-grain premium hunting bullets (bonded or controlled expansion). With a 1:8 twist, it delivers balanced energy, flatter trajectories, and reliable cycling in an AR-15.

8.6 Blackout:

Excels with 190–225-grain bonded or solid copper bullets for deeper penetration. You’ll want to tune the gas and verify bullet stability in your twist rate to prevent jacket failure at high RPMs.

Winner: .338 ARC for reach and consistency, 8.6 BLK for short-range shock.

Precision / Match Use

.338 ARC: With 165–180-grain match bullets, it becomes a “big-bore Grendel” that holds velocity well past 500 yards. Reliable feeding, low recoil, and high consistency.

8.6 Blackout: It can shoot accurately, but its strength is terminal energy, not tight group precision. It’s not a match round it’s a close-range powerhouse.

Platform Comparison: AR-15 vs. AR-10

.338 ARC Platform (AR-15)

• Bolt: Uses a 6.5 Grendel Type II bolt face.

• Mags: Standard Grendel/ARC magazines feed reliably.

• Gas System: Mid-length or rifle gas for smooth cycling.

• Weight: Lighter and more portable.

• Best Barrel Length: 16–18 inches.

Why Choose It: If you already own an AR-15 or want a rifle that’s easy to carry, cheap to build, and versatile from match shooting to hunting this is your caliber.

8.6 Blackout Platform (AR-10 or Bolt)

• Bolt: Uses standard .308/6.5 Creedmoor bolts.

• Mags: Works in standard AR-10/6.5CM magazines.

• Gas: Requires tuning adjustable gas block or buffer setup recommended.

• Twist Rate: 1:3–1:4 to stabilize heavy subs.

• Best Barrel Length: 8–12 inches (suppressed).

Why Choose It: If you want a quiet, compact rifle that hits like a hammer inside 200 yards, this is the specialist’s choice.

Mechanical Concerns: Bolt Lugs & High-RPM Issues

Bolt lug concerns for AR-15s running .338 ARC

The larger Grendel/ARC case head increases bolt thrust compared with 5.56, so AR-15 bolts and lug webs take more stress; common failures come from uneven lug contact, thin web geometry, brittle heat treatments, or hot/overpressure handloads. Inspect bolt lug faces and the web for rounding, cracks, or mushrooming; if you see damage, retire the bolt immediately, verify lug timing and headspace, and replace with a forged high-spec bolt from a reputable vendor. Regularly chronograph loads and avoid exceeding published MAPs proactive inspection intervals and proper material choices greatly reduce risk.

Key AR-10 / 8.6 Blackout issues

The 8.6 Blackout’s extreme twists (1:3–1:4) and short, suppressed barrels introduce two practical hazards: bullet integrity at very high RPM (some jackets or cup-and-core bullets can fragment under extreme spin) and finicky gas/functional tuning in short-barrel + can setups. Mitigation is straightforward use bullets proven for high-RPM or monolithic/bonded designs, start testing with a sacrificial muzzle device before using an expensive suppressor, and fit an adjustable gas block or tuned carrier/buffer so the rifle will run reliably on both subsonic and supersonic loads. Inspect your muzzle device and suppressor baffles early in break-in.

Redleg Reloading Sheets (Free Download)

We built free reloading and load development sheets for precision shooters. Track your brass life, powder charges, chronograph results, and group data in one easy Excel file.

👉 Download Redleg Reloading Sheets

Test at least three powders and three seating depths for each load precision is built, not guessed.

Final Thoughts

.338 ARC vs 86 Blackout Both cartridges redefine what “big-bore” means in a compact rifle.

• The .338 ARC is the versatile precision choice, offering balanced power, reliability, and easy parts availability.

• The 8.6 Blackout is the suppressed specialist, built to deliver controlled chaos in short barrels.

If you’re chasing balance, stay with 338 ARC. If you’re chasing shock factor, go 8.6 Blackout.

Either way, Redleg Company can help you design, build, and tune the rifle that brings out the best in each round.

Contact Redleg Company

Want help building or tuning one of these rigs, or ready to book a gunsmithing consult? Reach out to Redleg Company in Chandler, MN we build custom rifles, tune gas systems, and support load development.

Email us at info@redlegguns.com, call or text (507) 677-6007, or start a build request at https://www.redlegguns.com.

Tell us which cartridge you’re thinking about (.338 ARC or 8.6 Blackout) and we’ll set up a free 15-minute consult to go over options and pricing.