338 ARC subsonic ammunition puts a heavy .338-caliber bullet below the speed of sound in an AR15-size cartridge, so a suppressed rifle loses the supersonic crack and keeps real energy on target. Hornady introduced the 338 ARC around its 307 gr Sub-X subsonic load, rated at 1,050 fps and 752 ft-lb from a 16-inch barrel. Chattahoochee Munitions manufactures complete 338 ARC firearms for short-barrel subsonic shooting: the CM7S bolt action in 8, 12.5 and 16-inch barrels and the CM15 AR15 in 8.5, 10.5 and 16-inch barrels.
What Is 338 ARC?
The 338 ARC (Advanced Rifle Cartridge) is a .338-caliber cartridge from Hornady’s ARC family, sized for the AR15. SAAMI published its 338 ARC specification in January 2025.
| Specification | 338 ARC |
|---|---|
| Bullet diameter | .338 inch |
| Case head | .441 inch, the same ARC and 6.5 Grendel family head |
| Case length | 1.300 inch |
| Maximum overall length | 2.260 inch (AR15 magazine length) |
| Maximum average pressure | 52,000 psi |
| SAAMI test barrel | 16 inch, 1:8 twist |
| Bolt face | 6.5 Grendel Type II |
Why Does Subsonic Matter?
Sound travels at roughly 1,100 to 1,125 fps at sea level, depending on air temperature. A bullet faster than that makes a sonic crack along its whole flight path, and no suppressor can remove it. A suppressor reduces muzzle blast. Subsonic ammunition removes the crack. Together they give the low report that suppressed hunting and training rely on.
The cost of subsonic is velocity, so energy has to come from bullet weight. That is the point of 338 ARC subsonic: a 307 gr bullet is far heavier than common AR15 subsonic bullets, while the cartridge still fits an AR15 magazine.
Factory 338 ARC Subsonic Ammunition
Hornady’s published figures for its factory 338 ARC loads:
| Load | Type | Muzzle velocity | Muzzle energy |
|---|---|---|---|
| 307 gr Sub-X | Subsonic, expanding | 1,050 fps | 752 ft-lb |
| 175 gr HP Match | Supersonic | 2,075 fps | 1,673 ft-lb |
Hornady rates 338 ARC subsonic at about 1.5 times the energy of 300 Blackout subsonic. Hornady also offers 170 gr MonoFlex and 215 gr FTX supersonic hunting loads and a 285 gr FMJ. Chattahoochee Munitions manufactures firearms for this cartridge and does not load 338 ARC ammunition.
Twist Rate for Heavy Subsonic Bullets
Long, slow bullets need more spin. SAAMI lists a 1:8 twist, and that suits the 175 gr supersonic load. Chattahoochee Munitions uses faster twists for the 307 gr bullet:
- CM7S 12.5 and 16 inch: 1:6
- CM7S 8 inch: 1:5
- CM15 8.5 and 10.5 inch: 1:5
- CM15 16 inch: 1:8
The full explanation, with spin rates, is in 338 ARC twist rate.
Barrel Length for Subsonic Loads
A subsonic load is capped by the speed of sound, not by barrel length, so a short barrel gives up little. In Chattahoochee Munitions testing, 338 ARC powder use is largely complete around 12 inches. That is why the 12.5-inch CM7S and the 10.5-inch CM15 are the center of the lineup, with 8 and 8.5-inch options for a shorter suppressed host and 16-inch options for supersonic loads. See 338 ARC barrel length.
Choosing a Suppressor
The cartridge is made to be shot suppressed. For subsonic ammunition, an enclosed suppressor is recommended on both the CM7S bolt action and the CM15 semi-automatic. With supersonic ammunition, the bolt action stays with an enclosed suppressor, and a flow-through suppressor is recommended on the semi-automatic. The PLACID 375 is an enclosed suppressor rated from 5.56 NATO through .375. A suppressed firearm is still not hearing safe; NIOSH sets the recommended exposure limits, so wear hearing protection. Details are in 338 ARC suppressor.
Bolt Action or AR15?
Both platforms use the 6.5 Grendel Type II bolt face.
- CM7S bolt action: Manually operated, so it adds no action noise and has no gas to tune. Mos-Tek 416R stainless barrels with 5R rifling, a TriggerTech R700 Primary trigger, the CM7 billet chassis and a 10-round AICS-pattern magazine.
- CM15 AR15: Semi-automatic with a Superlative Arms adjustable gas block and an Odin Works adjustable buffer, so it can be tuned for subsonic, supersonic and suppressed use. It ships with a 10-round 338 ARC AR15 magazine.
Read 338 ARC bolt action vs semi auto for the full comparison.
How 338 ARC Compares
- 338 ARC vs 300 Blackout: same AR15 envelope, heavier .338 bullet.
- 338 ARC vs 8.6 Blackout: same bullet diameter, AR15 vs AR10-size cartridge.
- 375 Raptor vs 338 ARC: the next step up in subsonic bullet weight.
Subsonic Firearms From Chattahoochee Munitions
Every Chattahoochee Munitions 338 ARC barrel is 416R stainless with an E100 Blackout Cerakote finish and 5/8×24 muzzle threads.
- CM7S 338 ARC 12.5-inch bolt action pistol or SBR
- CM7S 338 ARC 8-inch bolt action pistol or SBR
- CM7S 338 ARC 16-inch bolt action rifle
- CM15 338 ARC 10.5-inch AR15 pistol or SBR
- CM15 338 ARC 16-inch AR15 rifle
See every configuration, barreled action and magazine in Containment Zone 338.
338 ARC Subsonic FAQ
Who makes a 338 ARC subsonic rifle?
Chattahoochee Munitions manufactures complete 338 ARC firearms: the CM7S bolt action in 8, 12.5 and 16-inch barrels, and the CM15 AR15 in 8.5, 10.5 and 16-inch barrels, in pistol, SBR and rifle configurations.
How fast is 338 ARC subsonic?
Hornady rates its 307 gr Sub-X at 1,050 fps and 752 ft-lb from a 16-inch barrel, just under the speed of sound.
Is 338 ARC subsonic better than 300 Blackout subsonic?
It carries more energy. Hornady rates 338 ARC subsonic at about 1.5 times the energy of 300 Blackout subsonic, because the bullet is heavier and wider at similar velocity.
What twist rate does a subsonic 338 ARC need?
Chattahoochee Munitions uses 1:6 on the 12.5 and 16-inch CM7S, 1:5 on the 8-inch CM7S and the 8.5 and 10.5-inch CM15, and 1:8 on the 16-inch CM15.
What barrel length is right for 338 ARC subsonic?
About 12 inches. Chattahoochee Munitions testing shows powder use is largely complete there, so 10.5 and 12.5-inch barrels give a compact suppressed host.
Does 338 ARC run in a standard AR15?
It runs in an AR15 with a 338 ARC barrel, a 6.5 Grendel Type II bolt and a 338 ARC magazine. The CM15 is manufactured that way from the factory.
Can a bolt action shoot 338 ARC subsonic?
Yes. The CM7S bolt action is manually operated, so subsonic ammunition never has to cycle the action, and it adds no action noise when suppressed.
Is suppressed 338 ARC subsonic hearing safe?
No. A suppressed subsonic shot is much quieter than an unsuppressed shot, but it is not hearing safe. Wear hearing protection.
Does Chattahoochee Munitions sell 338 ARC ammunition?
No. Chattahoochee Munitions manufactures 338 ARC firearms, barreled actions, uppers and magazines. Factory 338 ARC ammunition is loaded by Hornady.