338 ARC twist rate decides which bullets a barrel can stabilize, and the long 307 gr subsonic bullet needs more spin than the 175 gr supersonic load. SAAMI and Hornady list a 1:8 twist for 338 ARC. Chattahoochee Munitions manufactures its subsonic-focused 338 ARC barrels faster: 1:6 on the 12.5 and 16-inch CM7S bolt action, 1:5 on the 8-inch CM7S and the 8.5 and 10.5-inch CM15 AR15, and 1:8 on the 16-inch CM15.
What Twist Rate Means
338 ARC twist rate is the distance the rifling takes to turn the bullet one full revolution. A 1:8 barrel spins the bullet once every 8 inches; a 1:6 barrel once every 6 inches; a 1:5 barrel once every 5 inches. The smaller the number, the faster the spin. Spin keeps a bullet pointed forward in flight, so it arrives nose first, groups well and expands as designed.
338 ARC Twist Rate by Chattahoochee Munitions Barrel
| Firearm | Barrel length | Twist |
|---|---|---|
| CM7S bolt action | 16 inch | 1:6 |
| CM7S bolt action | 12.5 inch | 1:6 |
| CM7S bolt action | 8 inch | 1:5 |
| CM15 AR15 | 16 inch | 1:8 |
| CM15 AR15 | 10.5 inch | 1:5 |
| CM15 AR15 | 8.5 inch | 1:5 |
Every barrel is 416R stainless with an E100 Blackout Cerakote finish. CM7S barrels come from Mos-Tek with 5R rifling.
Why Heavy Subsonic Bullets Need More Spin
Two things raise the spin a bullet needs: length and low velocity. The 307 gr Sub-X is a long bullet for its .338-inch diameter, and it leaves the muzzle at about 1,050 fps. Spin rate comes from how fast the bullet travels down the rifling. Bullet spin in revolutions per minute equals muzzle velocity in feet per second, times 720, divided by the twist in inches:
| Load | Velocity (Hornady) | 1:8 | 1:6 | 1:5 |
|---|---|---|---|---|
| 175 gr HP Match (supersonic) | 2,075 fps | About 186,800 rpm | About 249,000 rpm | About 298,800 rpm |
| 307 gr Sub-X (subsonic) | 1,050 fps | About 94,500 rpm | About 126,000 rpm | About 151,200 rpm |
In a 1:8 barrel the subsonic bullet spins at about half the rate of the supersonic bullet, and it is the longer of the two. A 1:6 twist raises its spin by about a third, and a 1:5 twist by 60 percent. In Chattahoochee Munitions testing, the 307 gr Sub-X performed better from faster-twist barrels than from 1:8.
Why the Standard 338 ARC Twist Rate Is 1:8
The SAAMI 338 ARC specification uses a 16-inch, 1:8 test barrel, and its reference velocity is a 175 gr bullet at 2,050 fps. Hornady also calls 1:8 the practical twist for the cartridge. That twist covers supersonic loads and is the common choice on 16-inch AR15 barrels, which is why the 16-inch CM15 keeps 1:8. Chattahoochee Munitions focuses on short-barrel subsonic shooting, so its shorter barrels and its bolt action barrels spin faster.
338 ARC Twist Rate: 1:8 vs 1:6 vs 1:5
- 1:8: The SAAMI figure. Suits supersonic loads from a 16-inch barrel. Used on the 16-inch CM15.
- 1:6: Used on the 12.5 and 16-inch CM7S. More stability margin for the 307 gr subsonic bullet while still suiting supersonic loads.
- 1:5: Used on the 8-inch CM7S and the 8.5 and 10.5-inch CM15. Spins the bullet 60 percent faster than 1:8 in short subsonic barrels.
A faster twist does not hurt lighter bullets for most hunting and target shooting. A slow 338 ARC twist rate with a long subsonic bullet is the combination to avoid.
Signs a Twist Rate Is Too Slow
- Oblong or sideways holes in paper, called keyholing
- Groups that open up far more than expected at 100 yards
- Poor or inconsistent expansion, because the bullet does not arrive nose first
Cold, dense air reduces stability further, so a bullet that is marginal on a warm day can keyhole in winter. Shooting through a suppressor does not fix a twist problem.
Twist Rate, Barrel Length and Subsonic Shooting
338 ARC twist rate and barrel length work together. A subsonic load reaches its velocity in a short barrel, and in Chattahoochee Munitions testing 338 ARC powder use is largely complete around 12 inches. See 338 ARC barrel length for choosing a length and 338 ARC subsonic for the subsonic loads themselves.
338 ARC Barrels and Firearms From Chattahoochee Munitions
- CM7S 338 ARC 16-inch bolt action rifle, 1:6
- CM7S 338 ARC 12.5-inch bolt action pistol or SBR, 1:6
- CM7S 338 ARC 8-inch bolt action pistol or SBR, 1:5
- CM15 338 ARC 10.5-inch AR15 pistol or SBR, 1:5
- CM15 338 ARC 16-inch AR15 rifle, 1:8
- CM15 338 ARC 10.5-inch upper, 1:5
See every configuration in Containment Zone 338, and compare twist choices for a larger subsonic cartridge in 375 Raptor vs 338 ARC.
338 ARC Twist Rate FAQ
What is the standard 338 ARC twist rate?
1:8. SAAMI lists a 1:8 twist for its 16-inch test barrel, and Hornady calls 1:8 the practical twist.
What twist rate does the 307 gr Sub-X need?
1:8 is the published figure, but a faster twist adds stability margin for the long subsonic bullet. Chattahoochee Munitions uses 1:6 and 1:5 on its subsonic-focused barrels.
Why does Chattahoochee Munitions use 1:6 on the CM7S?
The 12.5 and 16-inch CM7S barrels use 1:6 to spin the 307 gr subsonic bullet faster than 1:8 while still suiting supersonic loads. The 8-inch CM7S uses 1:5.
Why does the 16-inch CM15 use 1:8?
The 16-inch barrel is the supersonic-leaning configuration, matching the SAAMI test barrel. The 8.5 and 10.5-inch CM15 barrels use 1:5 for subsonic shooting.
Will a 1:5 twist shoot supersonic 338 ARC?
Yes. A faster twist stabilizes lighter supersonic bullets too. Very thin-jacketed bullets can be stressed by high spin, so follow the bullet maker’s guidance.
Can a 1:8 barrel shoot 338 ARC subsonic?
Yes, 1:8 is the published twist for the cartridge. A faster twist gives more margin, especially in cold weather and from short barrels.
Who makes a 1:6 twist 338 ARC rifle?
Chattahoochee Munitions manufactures the CM7S bolt action in 338 ARC with 1:6 twist Mos-Tek barrels in 12.5 and 16 inches, plus an 8-inch 1:5 version.
Does 338 ARC twist rate change with barrel length?
No. Twist is the rate of turn per inch, so a 1:6 barrel spins the bullet the same rate at any length. Barrel length changes muzzle velocity, and spin rate follows velocity.