CM Ammunition – 308 Winchester – 300gr MAKER REX Expanding Subsonic – 20rd Boxes

Warning: This 308 subsonic ammo requires a 1:5 twist barrel. It will not stabilize in 1:8 or 1:10 twist barrels, and it will not cycle standard 308 AR10 rifles, including CM10 16″ and 18″ configurations. The CM10 SUBSONIC cycles it with an enclosed suppressor attached. Fire it only in a firearm specified for 300-grain subsonic 308 Winchester loads.

CM 308 subsonic ammo is loaded with the 300-grain Maker Bullets 300gr .308 REX, a CNC-machined, lead-free expanding bullet, seated in new Starline 308 Winchester brass. From a 12.5″ 1:5 twist barrel it averages 1,005 fps, below the speed of sound, which keeps the report down to the action and muzzle when fired through a suppressor.

The load was developed alongside the CM firearms in District 308, CM’s 308 Winchester system, and is matched to the 1:5 twist barrels used in its subsonic configurations.

Specifications

• Cartridge: 308 Winchester
• Bullet: 300gr Maker REX, CNC-machined, lead-free, expanding
• Bullet diameter: .308″
• Bullet length: 2.06″
• Ballistic coefficient: .40 (G1, Maker published)
• Brass: New Starline 308 Winchester
• Cartridge overall length: 2.79″
• Average muzzle velocity: 1,005 fps (12.5″ barrel, 1:5 twist)
• Velocity standard deviation: 2.44 fps (10-round sample)
• Required twist rate: 1:5
• Quantity: 20 rounds per box

CM Firearms Compatible with This 308 Subsonic Ammo

Compatible: 12.5″ barrels with 1:5 twist

These configurations use 1:5 twist barrels that stabilize the 300-grain REX.

• CM10 308 Win SUBSONIC 12.5″ AR10 Pistol or SBR (Cobalt Kinetics Green, Black Tungsten, or FDE Metallic): cycles this load with an enclosed suppressor attached
• CM7S 308 Win 12.5″ Bolt Action Short Barrel Rifle – WOOX Walnut
• CM7S 308 Win 12.5″ Bolt Action Pistol or SBR – FDE Metallic
• CM7S 308 Win 12.5″ Bolt Action Pistol or SBR – Cobalt Kinetics Green and Coyote
• CM7S 308 Win 12.5″ Bolt Action Pistol or SBR – Black Tungsten

Not compatible

• CM7S 16″ bolt-action rifles (1:8 twist): the twist is too slow to stabilize a 300-grain bullet at subsonic velocity.
• CM10 16″ and 18″ rifles and CM10 12.5″ supersonic configurations (1:10 twist): the twist will not stabilize the bullet, and the gas system will not cycle a subsonic load.
• Other manufacturers’ AR10 rifles: most will not cycle subsonic 308 Winchester, and CM has not verified this load in any AR10 other than the CM10 SUBSONIC.

Why 308 Subsonic Ammo Requires a 1:5 Twist

Stability depends on bullet length and velocity. A conventional supersonic 308 Winchester load uses a 150 to 175-grain bullet at roughly 2,600 fps or more, and a 1:10 or 1:8 twist stabilizes it easily. The 300-grain REX is 2.06″ long and leaves the muzzle near 1,005 fps, so it needs far more spin per inch of travel. The 1:5 twist provides that spin. In a slower-twist barrel the bullet can yaw in flight, which degrades accuracy and can cause a baffle strike in a suppressor.

Why a Standard AR10 Will Not Cycle It

A 308 AR10 is gassed to run on the pressure and dwell time of supersonic ammunition. A subsonic load produces far less gas, so a standard rifle will not reliably cycle it. The CM10 SUBSONIC was configured specifically to cycle this class of load, and it does so with an enclosed suppressor attached. That configuration also carries the opposite limitation: supersonic ammunition in a CM10 SUBSONIC is very problematic and potentially dangerous. Keep supersonic and subsonic 308 Winchester separate, and match the ammunition to the firearm it was specified for.

Range Card

Reference data for this 308 subsonic ammo at its 1,005 fps average muzzle velocity. Elevation is shown for a 100-yard zero; windage is shown with no crosswind.

The heavy bullet retains velocity well, losing about 100 fps over 500 yards, but its drop is steep. Past 100 yards, confirm your dope on paper before taking a shot.

308 Subsonic Ammo FAQ

Will this 308 subsonic ammo cycle my AR10?

Only if it is a CM10 SUBSONIC, which cycles it with an enclosed suppressor attached. Standard 308 AR10 rifles, including CM10 16″ and 18″ configurations, are gassed for supersonic ammunition and will not reliably cycle a 300-grain subsonic load. CM has not verified this load in any other manufacturer’s AR10.

Can I shoot it in a 1:8 or 1:10 twist bolt-action rifle?

No. The 2.06″ 300-grain REX needs a 1:5 twist to stabilize at subsonic velocity. In a 1:8 or 1:10 barrel it can yaw in flight, which hurts accuracy and can cause a baffle strike in a suppressor.

Do I need a suppressor to shoot it?

In the CM10 SUBSONIC, the load cycles with an enclosed suppressor attached. In the CM7S 12.5″ bolt-action configurations, it fires and feeds normally with or without one, though a suppressor is where a subsonic load earns its place, with no supersonic crack downrange and a report that is mostly the muzzle and the action.

How much does it drop past 100 yards?

With a 100-yard zero, the load drops about 34.6″ at 200 yards and 106.6″ at 300 yards. See the range card above, and confirm your own data on paper before taking longer shots.

Choosing This Load

This 308 subsonic ammo is the right choice for a CM 12.5″ 1:5 twist firearm run with a suppressor, where a quiet report and a heavy expanding bullet matter more than flat trajectory. If your 308 Winchester has a 1:8 or 1:10 twist, or is a standard AR10, use supersonic ammunition instead. To compare subsonic and supersonic configurations across bolt-action and semi-automatic platforms, see District 308.