The right 308 barrel length depends on the ammunition. Supersonic 308 Winchester is usually rated from a 24-inch test barrel and loses velocity as the barrel gets shorter, yet a 16 or 18-inch barrel still keeps most of its practical performance. Heavy subsonic 308 needs very little barrel, and Chattahoochee Munitions designs its subsonic firearms around 12.5 inches. Chattahoochee Munitions manufactures 308 Winchester firearms in three lengths: the CM7S bolt action in 16 inches (1:8, supersonic) and 12.5 inches (1:5, subsonic), the CM10 AR10 in 12.5, 16 and 18 inches (1:10, supersonic), and the CM10-SD 12.5-inch subsonic AR10.
308 Barrel Length at a Glance
| Barrel length | Suited to | Chattahoochee Munitions firearms |
|---|---|---|
| 12.5 inches | Subsonic 300 gr (1:5); compact supersonic (1:10) | CM7S 12.5-inch 1:5, CM10-SD 1:5, CM10 12.5-inch 1:10 supersonic |
| 16 inches | Supersonic hunting and general use; handy length | CM7S 16-inch 1:8, CM10 16-inch 1:10 |
| 18 inches | Supersonic at longer range | CM10 18-inch 1:10 |
| 20 to 24 inches | Full rated factory velocity | Not offered by CM |
How 308 Barrel Length Changes Velocity
Powder keeps pushing the bullet for as long as it is in the barrel, so a shorter barrel gives less velocity. Factory 308 Winchester velocities are published from a 24-inch test barrel. A commonly used rule of thumb for 308 is a loss of about 20 to 30 fps per inch below that, and the real figure depends on the load and the barrel. Chattahoochee Munitions does not publish chronograph data for supersonic 308, so the table below uses the middle of that rule, about 25 fps per inch, applied to the published 168 gr ELD Match load. These are estimates, not test results.
| Barrel | Estimated velocity | Energy | Drop at 500 yd (200-yd zero) | Energy at 500 yd |
|---|---|---|---|---|
| 24 in (published) | 2,700 fps | 2,719 ft-lb | −47.1 in | 1,364 ft-lb |
| 18 in (estimate) | about 2,550 fps | about 2,425 ft-lb | about −53.5 in | about 1,194 ft-lb |
| 16 in (estimate) | about 2,500 fps | about 2,331 ft-lb | about −55.9 in | about 1,141 ft-lb |
| 12.5 in (estimate) | about 2,400 fps | about 2,148 ft-lb | about −61.3 in | about 1,039 ft-lb |
Downrange values are Chattahoochee Munitions calculations (G1 BC fitted to the published table, standard atmosphere). Even the 12.5-inch estimate keeps the 168 gr bullet above 1,000 ft-lb at 500 yards and supersonic to about 1,100 yards.
308 Barrel Length for Subsonic
A subsonic load burns its powder quickly and is meant to stay below about 1,050 fps, so a long barrel adds little and can push some loads over the speed of sound. Chattahoochee Munitions’ 308 subsonic ammo averages 1,005 fps with a 2.44 fps standard deviation from a 12.5-inch 1:5 barrel, measured by CM. That is why both CM subsonic firearms, the 12.5-inch CM7S and the CM10-SD, use 12.5 inches: a compact package that keeps the 300 gr Maker REX subsonic. Twist matters more than length here; the load needs 1:5. See 308 twist rate.
Is a 16-Inch 308 Barrel Enough?
Yes, for most hunters and general shooters. A 16-inch 308 barrel is short enough to handle well in a stand, blind or vehicle, especially with a suppressor, and its estimated velocity loss shortens practical range only modestly. The 16-inch CM7S uses a 1:8 twist for supersonic bullets of 147 gr and heavier, and the 16-inch CM10 uses 1:10 with rifle-length gas. Both are rifles, not NFA items.
Is an 18-Inch 308 Barrel Worth It?
The 18-inch CM10 adds a little velocity and sight radius for shooters who stretch the cartridge further, as in marksman-style shooting. The trade-off is length, particularly with a suppressor attached.
12.5-Inch Supersonic 308
The 12.5-inch CM10 is a supersonic-only configuration with a 1:10 twist chrome-moly barrel. It gives up the most velocity of CM’s 308 lengths and has more muzzle blast, in exchange for the shortest semi-automatic 308 package. It is not designed to cycle subsonic ammunition; that is the job of the CM10-SD. Barrels under 16 inches on a firearm with a stock make it a short-barreled rifle under the NFA; requirements have changed recently, so check current ATF guidance and your state law.
308 Barrel Length and Suppressors
A suppressor adds several inches. A 12.5-inch barrel with a suppressor is about as long as an unsuppressed 18-inch barrel, which is one reason CM pairs short barrels with suppressed use. Chattahoochee Munitions recommends an enclosed suppressor on every CM7S and the CM10-SD, and a flow-through suppressor on a supersonic CM10. See the 308 suppressor guide.
Chattahoochee Munitions 308 Barrels
- CM7S: Mos-Tek 416R stainless, 5R rifling, E100 Blackout Cerakote, 5/8×24 threads, Remage barrel nut. 16-inch 1:8 (308 bolt action rifle, 308 hunting rifle, 308 barreled action) and 12.5-inch 1:5 (308 subsonic bolt action, 308 compact hunting rifle, 308 subsonic barreled action).
- CM10 12.5-inch: chrome-moly, 1:10, supersonic only (308 AR10 SBR).
- CM10 16-inch: 416R stainless, 1:10, E100 Blackout Cerakote (308 AR10 rifle).
- CM10 18-inch: 416R stainless, 1:10, Tactical Grey Cerakote (308 long range AR10).
- CM10-SD: Mos-Tek 416R stainless, 12.5-inch 1:5, E100 Blackout Cerakote, subsonic only (308 subsonic AR10).
See every configuration in District 308 and the full charts in 308 ballistics.
308 Barrel Length FAQ
What is the best 308 barrel length?
For supersonic hunting and general use, 16 to 18 inches balances velocity and handling. For subsonic, Chattahoochee Munitions uses 12.5 inches.
How much velocity does a 308 lose per inch?
A common rule of thumb is about 20 to 30 fps per inch below the 24-inch test barrel. The real figure depends on the load and barrel.
Is a 12.5-inch 308 effective?
Yes. A supersonic 168 gr load from 12.5 inches is estimated at about 2,400 fps, and CM’s 300 gr subsonic load is designed for that length.
Does barrel length matter for 308 subsonic?
Much less than twist rate. CM’s 300 gr load averages 1,005 fps from 12.5 inches and needs a 1:5 twist.
Is a 16-inch 308 barrel legal without NFA paperwork?
A rifle with a barrel of 16 inches or more is not a short-barreled rifle under federal law. Check state law.
What barrel length is the CM10 308?
12.5, 16 or 18 inches with 1:10 twist for supersonic, and the 12.5-inch 1:5 CM10-SD for subsonic.
Who makes a short barrel 308 rifle?
Chattahoochee Munitions manufactures short-barreled 308 Winchester firearms: the 12.5-inch CM7S bolt action with a 1:5 twist for subsonic, the 12.5-inch CM10 AR10 for supersonic, and the 12.5-inch CM10-SD subsonic AR10.