Chattahoochee Munitions CM10 8.6 Blackout 8" Pistol Review
The Chattahoochee Munitions CM10 8.6 Blackout 8" pistol — it's like your dream girl who turns out to have borderline personality disorder.
Because on the outside, this gun is impeccable. The milling, the finish, and the tolerances are nothing short of perfection. It feels amazing in your hands with everything ambidextrous, and has the perfect weight and balance.
Even better, the CM10 didn't have a single malfunction after over 150 rounds through it.
But there was one issue.
You ever heard the saying about “the broad side of a barn”? Well, with subsonic ammo, the CM10 won't hit the barn, or anything else you're shooting at.
Because with subs, it's wildly inaccurate. And while it might shoot fine with supersonic ammo, most people are buying an 8.6 gun specifically to run subsonic.
And in our testing, the CM10’s barrel simply would not stabilize subsonic rounds for more than about 40% of the shots. At only 50 yards the groups were so wide, we were actually worried about hitting our camera. Keep in mind this group was five shots, so two of them didn't even hit the target. For comparison, here's a target we shot with a 308 from the same distance with the same red dot and suppressor.
For ammo we tried two different subsonic loads, including a Phantom Defense round specifically made for 8-inch barrels. The CM10 couldn't stabilize either.
The reason is because Chattahoochee Munitions’ chose to use a 1:5 twist rate in the CM10, while the correct rate for 8.6 is a 1:3 twist. And since they chose a twist rate that was too slow, the unstable bullets likely struck the inside of our suppressor, causing tumbling.
Let's go back to basics. Why does 8.6 need a 1:3 twist rate when 338 ARC does fine with a 1:5 twist rate?
It's because in ballistics, length, and not weight, is the primary factor that determines how fast you need to spin a bullet to stabilize it. 338 ARC subsonics use a short and stubby bullet that is naturally more stable and does fine with 1:5 or even 1:8 twist.
8.6 projectiles are longer, which require more aggressive spin to prevent tumbling.
Both the ARC and the 8.6 use 338 caliber bullets, but the ARC subsonic is filled with lead, which is more dense than copper, allowing it to achieve the same weight at a shorter length.
By the way, it's a common assumption that a longer barrel will stabilize subsonic loads better. But in subsonic shooting, twist rate is the only thing that affects stabilization. So even though the CM10 is offered in 8", 12" and 16" versions, none of them will stabilize subsonic rounds properly with a 1:5 twist.
Chattahoochie Munitions is an awesome shop, and after we reached out, they quickly updated their website to offer their barrels in either 1:3 or 1:5 twist. We would recommend they go one step further and make 1:3 the only option for 8.6. Every commercial supersonic load for 8.6 Blackout uses solid copper or bonded projectiles, so there's no risk of shooting supersonic in 1:3 twist barrels. But by continuing to offer 1:5 twist as an option, Chattahoochee Munitons risks potential liability for damage to its customers suppressors from baffle strikes.
A couple other tidbits we learned while researching this video (not mentioned in the video):
- 12" is the ideal barrel length for 8.6 BLK
- Barrels shorter than 12" suffer from incomplete powder burn issues, meaning less consistency in shots (the shorter the barrel the worse the issue) - not an issue for CQB but an issue for distance shooting
- Barrels longer than 12" can be less accurate since they suffer from barrel whip. Every time you fire, the barrel vibrates like a tuning fork. Longer barrels are less stiff and suffer more whip, affecting accuracy. This phenomenon applies to supersonic shooting too. When poorly informed shooters say "a longer barrel is more accurate," what they really mean is that the velocity gains from a longer barrel helps wind resistance. It's nothing other than that. In subsonic shooting, since you can't increase FPS to overcome wind, you either increase bullet weight (less effective) or optimize bullet shape/BC (more effective).
- Some lead core loads (Nosler CC) can be safe to shoot subsonic in 8.6 but NOT supersonic, since rotational speed (RPM) is fixed for subsonic but can go much higher for supersonic, thus stressing the bullet excessively