The right 308 twist rate depends on the bullet’s length and speed. Common supersonic 150 to 178 gr bullets are stable from 1:10 or even 1:12, but a long, heavy subsonic bullet needs far more spin. Chattahoochee Munitions manufactures 308 Winchester firearms in three twist rates for that reason: 1:10 on the CM10 AR10, 1:8 on the 16-inch CM7S bolt action, and 1:5 on the 12.5-inch CM7S and the CM10-SD for CM’s 300 gr subsonic load.
308 Twist Rate at a Glance
| Twist | What it stabilizes | Chattahoochee Munitions firearms |
|---|---|---|
| 1:12 | Light and mid-weight supersonic bullets, about 150 to 168 gr | Not used by CM |
| 1:10 | Common supersonic bullets up to about 180 to 200 gr | CM10 12.5, 16 and 18 inch |
| 1:8 | Supersonic bullets of 147 gr and heavier, including long, heavy and solid-copper designs | CM7S 16 inch, 16-inch barreled action |
| 1:5 | The 2.06-inch, 300 gr Maker REX at subsonic speed; heavy supersonic loads in the CM7S | CM7S 12.5 inch, 12.5-inch barreled action, CM10-SD |
The SAAMI drawing for 308 Winchester lists a 1:12 twist (see the ANSI/SAAMI centerfire rifle standard). Most factory rifles use 1:10 or 1:12.
What 308 Twist Rate Means
Twist rate is the distance the rifling takes to turn the bullet one full revolution. A 1:10 barrel turns it once every 10 inches; a 1:5 barrel turns it once every 5 inches, so at the same speed the bullet spins twice as fast. Spin keeps the bullet pointed forward. Too little spin lets it yaw or tumble, which ruins accuracy and, with a suppressor attached, can cause a baffle strike.
Why Bullet Length Matters More Than Weight
Stability depends mostly on how long the bullet is for its diameter, and on its speed. A solid-copper bullet is longer than a lead-core bullet of the same weight, so it may need a faster 308 twist rate. Speed matters because spin rate is muzzle velocity divided by twist. A subsonic bullet at 1,000 fps spins at well under half the rate of a supersonic bullet at 2,700 fps from the same barrel, and it is usually the longest bullet in the cartridge. Both effects push toward a faster twist.
308 Twist Rate for Subsonic Ammunition
Chattahoochee Munitions loads its 308 subsonic ammo with the 300 gr Maker REX, a CNC-machined, lead-free expanding bullet that is 2.06 inches long. Maker specifies a 1:5 twist for it. From CM’s 12.5-inch 1:5 barrel the load averages 1,005 fps. Do not fire it through a 1:8 or 1:10 barrel.
The table below shows calculated stability for that bullet and for common supersonic bullets. These are Chattahoochee Munitions estimates using the Miller stability rule in a standard atmosphere, not test results. A value of 1.4 or higher is the usual target; below 1.0 the bullet is not stable.
| Bullet (velocity) | 1:5 | 1:8 | 1:10 | 1:12 |
|---|---|---|---|---|
| 300 gr Maker REX, 2.06 in (1,005 fps) | 2.7 | 1.1 | 0.7 | 0.5 |
| 178 gr ELD-X (2,600 fps) | 7.8 | 3.1 | 2.0 | 1.4 |
| 168 gr ELD Match (2,700 fps) | 9.9 | 3.9 | 2.5 | 1.7 |
| 150 gr SST (3,000 fps) | 12.2 | 4.8 | 3.0 | 2.1 |
The 300 gr bullet is unstable from 1:10 and 1:12 and marginal from 1:8, while 1:5 gives it a comfortable margin. The supersonic bullets are stable from every rate listed, but stable is not the same as well matched: a light bullet spun far faster than it needs gives up accuracy, as the next section explains. Bullet lengths for the supersonic loads are typical values for those designs.
Can a 308 Twist Rate Be Too Fast?
Yes, for light bullets. A twist much faster than a bullet needs over-stabilizes it. An over-stabilized bullet has more spin drift, the sideways drift in the direction of the rifling that grows with range, and the extra spin magnifies small imbalances in the bullet into larger groups. Light, thin-jacketed bullets are the most affected. That is why Chattahoochee Munitions matches bullet weight to twist: 147 gr and heavier in the 1:8 CM7S, 175 gr and heavier for supersonic loads in the 1:5 CM7S, and lighter bullets in a 1:10.
Why the 16-Inch CM7S Uses 1:8
The 16-inch CM7S is CM’s supersonic 308 bolt action. A 1:8 twist stabilizes virtually the same supersonic ammunition as a 1:10, from 147 gr hunting bullets to long, heavy match and solid-copper bullets. Chattahoochee Munitions does not recommend bullets lighter than 147 gr in the 1:8, because the faster twist over-stabilizes them and adds spin drift; those suit a 1:10. Every CM7S barrel is a Mos-Tek 416R stainless barrel with 5R rifling and E100 Blackout Cerakote. The 1:8 is still too slow for CM’s 300 gr subsonic load; use a 1:5 CM7S for that. The 12.5-inch 1:5 CM7S is designed for subsonic, and it can also fire heavy supersonic 308 loads of 175 gr and heavier.
Why the CM10 Uses 1:10
The CM10 12.5, 16 and 18-inch rifles are designed for supersonic 308 Winchester and 7.62×51 NATO. A 1:10 twist stabilizes common supersonic bullets up to about 180 to 200 gr, the range most 308 semi-automatic shooters use. The CM10-SD is different: it pairs a 12.5-inch 1:5 barrel with an enclosed suppressor so it can stabilize and cycle the 300 gr subsonic load. None of the 1:10 CM10s cycles subsonic ammunition, and the CM10-SD is limited to subsonic ammunition: the modifications that let it cycle subsonic loads leave supersonic pressure too high for it.
Does Twist Rate Change Velocity?
Only slightly. A faster twist adds a little friction, but barrel length and the load decide velocity. The bigger effect of a fast twist is on accuracy with light bullets, through over-stabilization and spin drift. See 308 barrel length and 308 ballistics.
308 Twist Rate and Suppressors
A stable bullet is part of safe suppressor use. Confirm stability on paper before you fire any subsonic load through a suppressor. For which suppressor to use on each CM platform, see the 308 suppressor guide.
308 Firearms by Twist Rate
- 1:5 bolt action: CM7S 308 subsonic bolt action, CM7S 308 bolt action SBR, CM7S 308 bolt action pistol, CM7S 308 compact hunting rifle, 308 subsonic barreled action
- 1:5 semi-automatic: CM10-SD 308 subsonic AR10
- 1:8 bolt action: CM7S 308 bolt action rifle, CM7S 308 suppressor ready rifle, CM7S 308 overwatch rifle, CM7S 308 hunting rifle, 308 barreled action
- 1:10 semi-automatic: CM10 308 AR10 SBR, CM10 308 AR10 rifle, CM10 308 DMR rifle
See every configuration in District 308.
308 Twist Rate FAQ
What is the standard 308 twist rate?
The SAAMI drawing lists 1:12, and most factory 308 rifles use 1:10 or 1:12. Chattahoochee Munitions uses 1:10 on the CM10, 1:8 on the 16-inch CM7S and 1:5 on its subsonic firearms.
Is 1:10 or 1:12 better for 308?
1:10 covers a wider range of bullet lengths, up to about 180 to 200 gr. 1:12 suits 150 to 168 gr bullets.
What twist rate do I need for 308 subsonic?
It depends on the bullet. CM’s 300 gr Maker REX is 2.06 inches long and needs 1:5.
Can I shoot 308 subsonic in a 1:10 twist?
Not CM’s 300 gr load. The bullet is not stable from 1:10 and can strike a suppressor baffle. Lighter subsonic bullets may be stable; follow the ammunition maker’s minimum twist.
Can a 1:8 twist shoot light 308 bullets?
Chattahoochee Munitions recommends 147 gr and heavier in the 1:8 CM7S. Lighter bullets are over-stabilized, which adds spin drift and costs accuracy; they suit a 1:10. From 147 gr up, a 1:8 stabilizes virtually the same ammunition as a 1:10.
Can a 308 twist rate be too fast?
Yes, for light bullets. Over-stabilization increases spin drift and magnifies small bullet imbalances, so accuracy suffers. Match the bullet weight to the twist.
Can a 1:5 twist 308 shoot supersonic ammunition?
In the 12.5-inch CM7S bolt action, yes: heavy supersonic loads of 175 gr and heavier can be used. Lighter supersonic bullets are over-stabilized in a 1:5. The CM10-SD is limited to subsonic ammunition.
What twist rate is the CM10 308?
1:10 on the 12.5, 16 and 18-inch CM10, and 1:5 on the 12.5-inch CM10-SD subsonic AR10.
Who makes a 1:5 twist 308 rifle?
Chattahoochee Munitions manufactures 1:5 twist 308 Winchester firearms: the CM7S 12.5-inch bolt action in chassis and walnut configurations, the 12.5-inch CM7S barreled action, and the CM10-SD 12.5-inch subsonic AR10.