338 ARC vs 300 Blackout comes down to bullet weight versus simplicity. Both fit the AR15 and both are made for suppressed subsonic shooting. The 338 ARC fires a heavier, wider bullet, a 307 gr .338-inch Sub-X that Hornady rates at about 1.5 times the energy of 300 Blackout subsonic. The 300 Blackout runs on a standard AR15 bolt and standard magazines and has far more ammunition choices. Chattahoochee Munitions manufactures complete firearms in both cartridges, on the CM15 AR15 and the CM7S bolt action.
338 ARC vs 300 Blackout at a Glance
| Feature | 338 ARC | 300 Blackout |
|---|---|---|
| Bullet diameter | .338 inch | .308 inch |
| Case family | Hornady ARC, .441-inch head (6.5 Grendel family) | 5.56 / .223 Remington family |
| Maximum overall length | 2.260 inch | 2.260 inch |
| Heavy subsonic bullet | 307 gr Sub-X, 1,050 fps, 752 ft-lb (Hornady, 16 inch) | About 190 to 220 gr |
| Supersonic bullets | 170 to 215 gr; 175 gr at 2,075 fps (Hornady, 16 inch) | Typically 110 to 150 gr |
| AR15 bolt | 6.5 Grendel Type II bolt face | Standard 5.56 bolt |
| AR15 magazine | 338 ARC-specific magazine | Standard 5.56 magazines |
| SAAMI | Published January 2025 | Standardized |
| Chattahoochee Munitions twist | 1:5 (8.5 and 10.5-inch CM15), 1:8 (16-inch CM15), 1:6 or 1:5 (CM7S) | 1:5 (10.5-inch CM15) |
Which Hits Harder Subsonic?
The 338 ARC. In 338 ARC vs 300 Blackout subsonic terms, energy is the clearest difference. Below the speed of sound both cartridges run at about the same velocity, roughly 1,000 to 1,050 fps, so bullet weight decides energy. The 307 gr Sub-X carries 752 ft-lb at the muzzle from a 16-inch barrel. A 200 to 220 gr 300 Blackout subsonic bullet at similar speed carries about two thirds of that, and Hornady rates 338 ARC subsonic at about 1.5 times the energy of 300 Blackout subsonic. The .338-inch bullet also starts wider before it expands.
338 ARC vs 300 Blackout Supersonic
Both cartridges carry supersonic loads for flatter shooting. The 338 ARC’s 175 gr HP Match is rated at 2,075 fps and 1,673 ft-lb from 16 inches, and its supersonic hunting bullets run from 170 to 215 gr. The 300 Blackout’s supersonic loads use lighter .308 bullets. For deer and hogs at moderate range, the 338 ARC puts more bullet weight on target; the 300 Blackout offers a wider range of loads and lower cost per round.
Conversion and Compatibility
For an AR15 owner, 338 ARC vs 300 Blackout is a question of which parts change.
- 300 Blackout: Change the barrel. The bolt, buffer system and magazines stay standard 5.56 parts.
- 338 ARC: Change the barrel, the bolt (6.5 Grendel Type II bolt face) and the magazine. The lower receiver stays.
The CM15 is manufactured as a complete 338 ARC system from the factory: a Type II spec 9310 steel bolt in a low mass bolt carrier group, a Superlative Arms adjustable gas block and an adjustable buffer for tuning subsonic, supersonic and suppressed use, and a 10-round 338 ARC AR15 magazine.
Ammunition Choice and Cost
Ammunition is where 338 ARC vs 300 Blackout favors the older cartridge. The 300 Blackout has been standardized much longer and is loaded by many ammunition makers, so it is easier to find and usually costs less. The 338 ARC is newer; Hornady offers the 307 gr Sub-X, 175 gr HP Match, 170 gr MonoFlex, 215 gr FTX and 285 gr FMJ. Chattahoochee Munitions does not load 338 ARC ammunition.
Suppressor and Barrel Length
Both cartridges are made for short barrels and suppressors. With subsonic ammunition an enclosed suppressor is recommended on both the bolt action and the semi-automatic; with supersonic ammunition, a flow-through suppressor is recommended on the semi-automatic and an enclosed suppressor on the bolt action. A .338 suppressor bore fits both cartridges, while a .30-caliber suppressor fits only the 300 Blackout. In Chattahoochee Munitions testing, 338 ARC powder use is largely complete around 12 inches; see 338 ARC barrel length and 338 ARC suppressor.
338 ARC vs 300 Blackout: Which Should You Choose?
- Choose 338 ARC for more subsonic energy and a heavier, wider hunting bullet in an AR15-size firearm.
- Choose 300 Blackout for standard AR15 parts and magazines, more ammunition choices and lower ammunition cost.
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 300 Blackout in Sector 300, including the CM15 300 Blackout 10.5-inch SBR, and read 300 Blackout subsonic.
For more on 338 ARC, read 338 ARC subsonic, 338 ARC ballistics and 338 ARC vs 8.6 Blackout.
338 ARC vs 300 Blackout FAQ
Is 338 ARC better than 300 Blackout?
For subsonic energy, yes: Hornady rates 338 ARC subsonic at about 1.5 times the energy of 300 Blackout subsonic. 300 Blackout is simpler to convert and has more ammunition choices.
Can I convert a 300 Blackout AR15 to 338 ARC?
You need a 338 ARC barrel, a 6.5 Grendel Type II bolt and a 338 ARC magazine. The lower receiver can stay.
Do 338 ARC and 300 Blackout use the same magazine?
No. 300 Blackout uses standard 5.56 magazines. 338 ARC uses a magazine made for its wider case.
Do they use the same bolt?
No. 300 Blackout uses a standard 5.56 bolt. 338 ARC uses a 6.5 Grendel Type II bolt face.
338 ARC vs 300 Blackout: which is quieter suppressed?
Both are subsonic below about 1,100 fps, so neither makes a sonic crack with subsonic ammunition. The report depends mostly on the suppressor and the firearm.
Can one suppressor work on both?
Yes, if it is rated for .338 caliber or larger. A .30-caliber suppressor works on 300 Blackout only.
Is 338 ARC good for deer and hogs?
Yes. Hornady positions 338 ARC for hogs, deer and medium game, with expanding subsonic and supersonic hunting loads. See 338 ARC hunting.
Who makes 338 ARC and 300 Blackout rifles?
Chattahoochee Munitions manufactures complete firearms in both: CM15 AR15 rifles, pistols and SBRs and CM7S bolt actions in 338 ARC, and CM15 and CM7S firearms in 300 Blackout.