CM 308 Subsonic Ammo – 300 gr Maker REX Expanding – 20 Rounds

Chattahoochee Munitions loads its 308 Subsonic Ammo with a 300 gr Maker REX expanding bullet in new Starline brass for its 1:5 twist CM7S 12.5-inch bolt actions and the CM10-SD subsonic AR10.

WARNING: This 308 subsonic ammo requires a 1:5 twist barrel. Do not fire it through a 1:8 or 1:10 twist barrel. The projectile may not stabilize, creating a risk of poor accuracy and a suppressor baffle strike. This ammunition will not cycle a conventional 308 AR10. The CM10 SUBSONIC cycles it with an enclosed suppressor attached. Use this ammunition only in a firearm specified for 300-grain subsonic 308 Winchester loads.

CM 308 subsonic ammo combines the 300-grain Maker REX with new Starline 308 Winchester brass. The REX is a CNC-machined, lead-free, solid-copper expanding projectile designed to open at subsonic velocity while retaining nearly all of its original weight.

From a 12.5-inch barrel with a 1:5 twist, the load averages 1,005 feet per second with a standard deviation of 2.44 feet per second across a 10-round sample. That velocity remains below the speed of sound, eliminating the supersonic flight crack when the ammunition is fired through a suppressor.

This is specialized ammunition for the 1:5 twist subsonic firearms found in District 308. It is not a reduced-recoil substitute for conventional 308 Winchester ammunition and should not be used in a firearm selected only because it is chambered in 308 Winchester.

308 Subsonic Ammo Specifications

• Cartridge: 308 Winchester
• Projectile: 300-grain Maker REX
• Construction: CNC-machined, lead-free, solid-copper expanding projectile
• Projectile diameter: .308 inch
• Projectile length: 2.06 inches
• Sectional density: .331
• Ballistic coefficient: .40 G1, as published by Maker Bullets
• Brass: New Starline 308 Winchester
• Cartridge overall length: 2.79 inches
• Average muzzle velocity: 1,005 fps from a 12.5-inch 1:5 twist barrel
• Velocity standard deviation: 2.44 fps from a 10-round sample
• Required barrel twist: 1:5
• Quantity: 20 rounds per box

Firearms Designed for This 308 Subsonic Ammo

CM developed this load around two types of 12.5-inch firearm with 1:5 twist barrels:

• CM7S subsonic bolt-action configurations: The manually operated action does not depend on ammunition pressure for cycling. These firearms can fire the load with or without a suppressor, although suppression is the application for which the ammunition was developed.
• CM10 SUBSONIC: This specialized semi-automatic configuration cycles the load with an enclosed suppressor attached. The suppressor supplies the additional backpressure required to operate the action.

Barrel twist, ammunition and operating system must be considered together. A firearm being chambered in 308 Winchester does not by itself make that firearm compatible with this load.

Do Not Use This Ammunition In

• CM7S 16-inch rifles with 1:8 twist barrels: The twist is too slow to stabilize the 2.06-inch projectile at subsonic velocity.
• CM10 16-inch and 18-inch rifles with 1:10 twist barrels: These barrels will not stabilize the projectile, and their gas systems are configured for supersonic ammunition.
• CM10 12.5-inch supersonic configurations with 1:10 twist barrels: Barrel length alone does not establish compatibility. These firearms use a different twist rate and operating configuration.
• Conventional AR10 rifles: Most are configured around the gas pressure produced by supersonic ammunition and will not cycle this load. CM has not verified it in another manufacturer’s AR10.

Why a 1:5 Twist Is Required

Projectile stability depends more heavily on length and velocity than weight alone. Conventional supersonic 308 Winchester ammunition normally uses a much shorter projectile traveling at considerably higher velocity. A 1:8 or 1:10 twist can stabilize those projectiles because their length and speed create a very different stability requirement.

The 300-grain Maker REX is 2.06 inches long and travels at approximately 1,005 feet per second. Its combination of extreme length and subsonic velocity requires substantially more rotational speed. A 1:5 twist supplies the rotation necessary to keep the projectile stable in flight.

An insufficient twist rate can allow the projectile to yaw or tumble. That produces poor accuracy on an unsuppressed firearm and can create a baffle or end-cap strike when a suppressor is attached. The barrel must demonstrate stable projectile flight before any ammunition is fired through a suppressor.

The 308 Winchester Subsonic Guide provides a more complete explanation of projectile stability, twist rate and the differences between CM’s subsonic and supersonic configurations.

Why Conventional AR10 Rifles Will Not Cycle It

A conventional 308 semi-automatic rifle uses the pressure and dwell time of supersonic ammunition to operate its action. A subsonic load produces much less gas pressure. Even if the projectile could be stabilized, a standard gas system would not normally receive enough energy to cycle reliably.

The CM10 SUBSONIC is configured specifically around this pressure curve. It uses a 1:5 twist barrel for stability and requires an enclosed suppressor to retain enough gas for semi-automatic operation.

A flow-through suppressor is generally preferred on a supersonic CM10 because it limits additional backpressure. That same characteristic works against the CM10 SUBSONIC, which needs additional backpressure to cycle. The 308 Suppressor Guide explains why CM recommends different suppressor designs for supersonic semi-automatics, subsonic semi-automatics and bolt actions.

Do not fire supersonic ammunition through the CM10 SUBSONIC. Keep supersonic and subsonic ammunition separate and match each load to the firearm for which it was specified.

Suppressed Performance

Subsonic ammunition removes the flight crack created when a projectile exceeds the speed of sound. A suppressor reduces the muzzle report, but it does not make the firearm silent. The remaining sound includes the firearm’s action, escaping gas and the projectile’s impact.

In a CM7S bolt action, the chamber remains closed until the shooter manually operates the bolt. This avoids the automatic action noise and gas release of a semi-automatic platform. In the CM10 SUBSONIC, the enclosed suppressor serves two purposes: it reduces the muzzle report and provides the backpressure necessary to cycle the action.

308 Subsonic Ammo Range Card

The following data reflects an average muzzle velocity of 1,005 feet per second and a 100-yard zero. Windage is shown with no crosswind. Use this information as a reference and confirm actual trajectory and point of impact with your firearm, optic, suppressor and environmental conditions.

The 300-grain projectile loses approximately 100 feet per second over 500 yards, but its trajectory becomes steep beyond the 100-yard zero. The range card should not replace verified data collected with the firearm and suppressor being used.

308 Winchester Guides

• 308 Winchester subsonic
• 308 twist rate
• 308 ballistics
• 308 for hunting
• 308 suppressor guide
• Every 308 configuration in District 308

308 Subsonic Ammo FAQ

Will this ammunition cycle my AR10?

It cycles the CM10 SUBSONIC with an enclosed suppressor attached. It will not reliably cycle conventional 308 AR10 rifles, including CM10 configurations designed for supersonic ammunition. CM has not verified this load in another manufacturer’s AR10.

Can I fire it through a 1:8 or 1:10 twist bolt-action rifle?

No. This 308 subsonic ammo uses a 2.06-inch projectile that requires a 1:5 twist at subsonic velocity. A slower twist can allow the projectile to yaw or tumble, degrading accuracy and creating the possibility of a suppressor strike.

Does the CM10 SUBSONIC require a suppressor?

Yes. The CM10 SUBSONIC requires an enclosed suppressor to provide the backpressure needed to cycle this load. A flow-through suppressor may release gas too efficiently to operate the action reliably.

Does a CM7S bolt action require a suppressor?

No. A compatible CM7S 12.5-inch 1:5 twist bolt action can fire the ammunition with or without a suppressor because its action is operated manually. A suppressor reduces the muzzle report and provides the intended application for the subsonic load.

How much does the load drop beyond 100 yards?

With a 100-yard zero, the reference data shows approximately 34.6 inches of drop at 200 yards and 106.6 inches at 300 yards. Confirm the actual trajectory with your firearm before relying on those values.

Who makes 308 subsonic ammo?

Chattahoochee Munitions loads this 308 subsonic ammo with a 300 gr Maker REX in new Starline brass, averaging 1,005 fps from a 12.5-inch 1:5 barrel, and manufactures the CM7S and CM10-SD firearms designed for it.

Choosing CM 308 Subsonic Ammo

Choose this 308 subsonic ammo for a CM 12.5-inch firearm with a 1:5 twist when suppressed performance and a heavy expanding projectile are more important than flat trajectory. Use it in a compatible CM7S bolt action or in the CM10 SUBSONIC with an enclosed suppressor attached.

If your firearm has a 1:8 or 1:10 twist barrel, or if it is a conventional AR10 configured for supersonic ammunition, this is not the correct load. Explore District 308 to compare CM’s subsonic and supersonic 308 Winchester systems.