338 ARC vs 8.6 Blackout compares two .338-caliber subsonic cartridges of different sizes. The 338 ARC fits the AR15 and fires a 307 gr subsonic bullet. The 8.6 Blackout is a larger .308 Winchester family cartridge, derived from the 6.5 Creedmoor case, that needs an AR10 or a short bolt action and carries heavier subsonic bullets of roughly 300 to 330+ gr. Chattahoochee Munitions manufactures complete firearms in both: 338 ARC on the CM15 AR15 and CM7S bolt action, and 8.6 Blackout on the CM10 AR10 and CM7S bolt action.
338 ARC vs 8.6 Blackout at a Glance
| Feature | 338 ARC | 8.6 Blackout |
|---|---|---|
| Bullet diameter | .338 inch | .338 inch |
| Case family | Hornady ARC, .441-inch head (6.5 Grendel family) | .308 Winchester family, from the 6.5 Creedmoor case |
| Maximum overall length | 2.260 inch (AR15 magazine) | AR10 / short action length |
| Heavy subsonic bullet | 307 gr Sub-X, 1,050 fps, 752 ft-lb (Hornady, 16 inch) | Roughly 300 to 330+ gr |
| Semi-automatic platform | CM15 AR15 | CM10 AR10 |
| Bolt action platform | CM7S, 6.5 Grendel Type II bolt face | CM7S, .308 bolt face |
| Chattahoochee Munitions barrel lengths | CM15: 8.5, 10.5, 16 inch; CM7S: 8, 12.5, 16 inch | 8, 12.5 and 16 inch |
| Chattahoochee Munitions twist | 1:5 or 1:6 (1:8 on the 16-inch CM15) | 1:3 or 1:5 |
| SAAMI | Standardized | Standardized |
The Real Difference: AR15 or AR10
338 ARC vs 8.6 Blackout is first a platform choice. Both fire .338-inch bullets, but the 338 ARC is sized for the AR15 magazine and its 2.260-inch overall length limit, while the 8.6 Blackout is a .308-family cartridge that needs a large-frame AR10 or a short-action bolt gun.
- 338 ARC: A lighter, more compact semi-automatic on the CM15 AR15, with AR15 lowers, controls and accessories.
- 8.6 Blackout: More case capacity and room for longer, heavier bullets, on the CM10 AR10 or the CM7S short action.
Which Hits Harder Subsonic?
The 8.6 Blackout, by bullet weight. Below the speed of sound both cartridges run at roughly the same velocity, so the heavier bullet carries more energy. The 338 ARC’s 307 gr Sub-X carries 752 ft-lb at 1,050 fps. An 8.6 Blackout subsonic bullet of 300 to 330+ gr at similar speed carries about the same to roughly 10 percent more, depending on the load. In 338 ARC vs 8.6 Blackout subsonic terms, the gap is small; the bigger difference is the firearm that carries it.
Supersonic Performance
Supersonic loads are part of the 338 ARC vs 8.6 Blackout decision too. The 338 ARC’s 175 gr HP Match is rated at 2,075 fps and 1,673 ft-lb from 16 inches, with 170 and 215 gr supersonic hunting loads. The 8.6 Blackout is mainly known as a subsonic cartridge, though its larger case also carries supersonic loads. For a 338 ARC owner, a single AR15 can run both subsonic and supersonic ammunition, with the CM15’s adjustable gas block and adjustable buffer tuned for each.
Twist Rate and Barrel Length
Heavy subsonic .338 bullets need a fast twist. Chattahoochee Munitions uses 1:6 on the 12.5 and 16-inch CM7S 338 ARC, 1:5 on the 8-inch CM7S and the short CM15s, and 1:8 on the 16-inch CM15. Its 8.6 Blackout barrels use 1:3 or 1:5; see 8.6 Blackout twist rate. Both cartridges are made for short barrels; in Chattahoochee Munitions testing, 338 ARC powder use is largely complete around 12 inches. See 338 ARC twist rate and 338 ARC barrel length.
Suppressors
For 338 ARC vs 8.6 Blackout suppressor choice, the bore size is the same. A suppressor rated for .338 caliber or larger fits both cartridges, such as the PLACID 375, an enclosed suppressor rated from 5.56 NATO through .375. With subsonic ammunition an enclosed suppressor is recommended on both bolt action and semi-automatic firearms. With supersonic ammunition, a flow-through suppressor is recommended on the semi-automatic, and the bolt action stays with an enclosed suppressor. A suppressed firearm is still not hearing safe; see the NIOSH noise guidance and wear hearing protection.
Bolt Action Versions
Chattahoochee Munitions manufactures both cartridges on the CM7S bolt action, with Mos-Tek 416R stainless barrels and a TriggerTech R700 Primary trigger. The 338 ARC CM7S uses a 6.5 Grendel Type II bolt face and a 10-round AICS-pattern magazine. A bolt action is manually operated, so it adds no action noise when suppressed and never needs subsonic ammunition to cycle it.
338 ARC vs 8.6 Blackout: Which Should You Choose?
- Choose 338 ARC for an AR15-size semi-automatic, lighter firearms and SAAMI factory ammunition from Hornady.
- Choose 8.6 Blackout for heavier subsonic bullets and more case capacity, if an AR10 or short action suits you.
Explore 338 ARC in Containment Zone 338, including the CM15 338 ARC 10.5-inch AR15 pistol or SBR and the CM7S 338 ARC 12.5-inch bolt action. Explore 8.6 Blackout in Area 8.6, including the CM7S 8.6 Blackout pistol, and read 8.6 Blackout subsonic.
More comparisons: 338 ARC vs 300 Blackout and 375 Raptor vs 338 ARC.
More on 8.6 Blackout: What Is 8.6 Blackout?, 8.6 Blackout ballistics, 8.6 Blackout ammo and 8.6 Blackout vs 300 Blackout.
338 ARC vs 8.6 Blackout FAQ
Is 338 ARC the same as 8.6 Blackout?
No. Both fire .338-inch bullets, but 338 ARC is an AR15-size cartridge and 8.6 Blackout is a larger .308-family cartridge that needs an AR10 or short action.
Can 8.6 Blackout run in an AR15?
No. 8.6 Blackout needs an AR10 such as the CM10, or a short bolt action such as the CM7S. 338 ARC is the AR15 option.
Which hits harder, 338 ARC or 8.6 Blackout?
8.6 Blackout carries somewhat more subsonic energy because its subsonic bullets are heavier. The 338 ARC’s 307 gr Sub-X carries 752 ft-lb.
Can they use the same bullets?
Both use .338-inch bullets, but each cartridge has its own overall length, so a bullet seated for one is not interchangeable ammunition for the other. Never fire one cartridge in the other’s chamber.
Can one suppressor work on both?
Yes. A suppressor rated for .338 caliber or larger works on both, such as the PLACID 375.
338 ARC vs 8.6 Blackout: which is better for a short barrel?
Both are made for short barrels. Chattahoochee Munitions offers 338 ARC down to 8 inches on the CM7S and 8.5 inches on the CM15, and 8.6 Blackout down to 8 inches.
Who makes 338 ARC and 8.6 Blackout rifles?
Chattahoochee Munitions manufactures complete firearms in both: CM15 AR15 and CM7S bolt action firearms in 338 ARC, and CM10 AR10 and CM7S bolt action firearms in 8.6 Blackout.