The short answer to 308 subsonic vs supersonic is that the two loads potentially ask for different firearms. A supersonic 308 Winchester load drives a 150 to 180-grain bullet well above the speed of sound and runs in nearly any 308. A subsonic 308 load stays below the speed of sound, and the twist it needs depends on bullet weight: a 1:5 twist stabilizes subsonic bullets of 220 grains and heavier, such as CM’s 300-grain load, while a 1:8 twist stabilizes subsonic bullets of 220 grains and lighter. Neither will cycle a standard AR10. A bolt action handles either load as long as the twist matches the bullet. A semi-automatic has to be configured for one or the other.
This guide covers the twist, gas and suppressor decisions behind the 308 subsonic vs supersonic choice, and how those decisions shape the CM7S and CM10 configurations in District 308.
What Subsonic Means for 308 Winchester
The speed of sound at sea level is about 1,125 fps at 59°F. It drops in cold air and rises in warm air, so a load that stays subsonic in January can sit closer to the line in August. A bullet traveling faster than sound produces a sonic crack along its entire flight path. A suppressor cannot remove that crack, because it happens downrange, not at the muzzle. That crack is the real dividing line in 308 subsonic vs supersonic.
Typical supersonic 308 Winchester factory loads push 150 to 175-grain bullets at roughly 2,600 to 2,800 fps from a 24″ test barrel, more than twice the speed of sound. CM 308 subsonic ammo uses a 300-grain Maker REX at a 1,005 fps average from a 12.5″ barrel. With no sonic crack, a suppressed shot leaves only the muzzle report and the sound of the action.
Both loads are 308 Winchester, a cartridge standardized by SAAMI. CM’s subsonic load is assembled to 308 Winchester dimensions with a 2.79″ cartridge overall length. What changes is bullet weight, velocity and the firearm needed to use them.
308 Subsonic vs Supersonic: Bullet Weight and Twist Rate
Stability depends on bullet length and velocity: a long bullet moving slowly needs far more spin per inch of barrel than a shorter bullet moving fast. Because every subsonic load travels near 1,000 fps, bullet weight, and with it bullet length, decides the twist.
Subsonic loads of 220 grains and heavier: 1:5 twist
A bullet traveling near 1,000 fps needs mass to carry useful energy, which is why CM’s subsonic load uses the 300-grain REX. A 1:5 twist stabilizes subsonic bullets of 220 grains and heavier; at 2.06″ long, the REX is well into that range. In a 1:8 or 1:10 barrel it yaws in flight, groups open up or keyhole, and in a suppressed firearm the bullet can strike a baffle.
Subsonic loads of 220 grains and lighter: 1:8 twist
Many subsonic 308 loads from other manufacturers use bullets of 220 grains and lighter. A 1:8 twist stabilizes those loads. They carry less energy than a heavier subsonic bullet, but they give a 1:8 twist rifle a subsonic option.
Supersonic loads: 1:10, 1:8 or 1:5
Supersonic 308 Winchester ammunition stabilizes in a 1:10 or 1:8 twist with no bullet-weight limitation from CM. A 1:5 barrel also fires supersonic ammunition, and CM recommends supersonic bullets of 160 grains and heavier in its 1:5 barrels.
308 Subsonic vs Supersonic in a Bolt Action or Semi-Automatic
Bolt action: twist is the only question
A bolt action has no gas system. The shooter cycles it, so pressure and gas volume never decide whether the next round feeds. In a bolt gun, the 308 subsonic vs supersonic question comes down to matching the twist rate to the bullet.
The CM7S 308 Winchester 12.5″ configurations use a 1:5 twist because the primary use of a short-barreled 308 is suppressed subsonic hunting. They fire subsonic bullets of 220 grains and heavier, including the 300-grain load, and supersonic ammunition of 160 grains and heavier. The CM7S 16″ rifles use a 1:8 twist, suited to supersonic ammunition and to subsonic loads of 220 grains and lighter, but not to stabilizing the 300-grain subsonic load.
Semi-automatic: the gas system decides
An AR10 cycles on gas bled from the barrel. A standard 308 AR10 is gassed for the pressure and volume of supersonic ammunition. A subsonic load produces far less gas, so most AR10 rifles on the market will not cycle it.
The CM10 SUBSONIC is configured specifically to cycle subsonic 308, and CM has verified it with CM’s 300-grain load and an enclosed suppressor attached. That configuration carries the opposite limitation: supersonic ammunition in a CM10 SUBSONIC is very problematic and potentially dangerous. The supersonic CM10 308 Winchester configurations in 12.5″, 16″ and 18″ use a 1:10 twist and a gas system set up for supersonic ammunition. They will not cycle subsonic loads.
That is why CM offers the semi-automatic 308 as two separate products instead of one rifle that claims to do both.
308 Subsonic vs Supersonic Safety: Match the Load to the Firearm
- Never fire supersonic ammunition in a CM10 SUBSONIC. It is very problematic and potentially dangerous.
- Never fire 300-grain subsonic ammunition in a 1:8 or 1:10 twist barrel. The bullet will not stabilize and can strike a suppressor baffle.
- Match subsonic bullet weight to twist. A 1:5 stabilizes subsonic bullets of 220 grains and heavier; a 1:8 stabilizes subsonic bullets of 220 grains and lighter.
- In a CM7S 1:5 barrel, use supersonic bullets of 160 grains and heavier.
- Keep subsonic and supersonic ammunition physically separate. Store, label and load them apart, especially if you own both a subsonic and a supersonic 308.
308 Subsonic vs Supersonic Suppressor Choice: Enclosed or Flow-Through
The right suppressor depends on the operating system as much as the load. An enclosed suppressor traps gas in a sealed baffle stack and provides maximum suppression. A flow-through suppressor vents gas forward and outward, which gives up some suppression in exchange for lower back pressure.
On a bolt action: enclosed for both loads
A CM7S has no gas system, so back pressure has no effect on how it cycles. That makes an enclosed suppressor the better choice on a bolt gun for both subsonic and supersonic ammunition: you get maximum suppression without any trade-off in function.
On a semi-automatic: match the suppressor to the configuration
On an AR10, back pressure changes how the rifle runs. The CM10 SUBSONIC cycles subsonic 308 with an enclosed suppressor attached. A supersonic CM10 is already gassed for supersonic pressure, so a flow-through suppressor is the better match there, because it keeps back pressure down.
For how the same questions play out in other cartridges, see our 300 Blackout subsonic and 8.6 Blackout subsonic guides, and the 300 Blackout gas system guide for a closer look at suppressor back pressure on a gas gun. Suppressors are regulated under the