BIG BULLETS: Ammo: 8.6 Blackout Review. “Unlike .300 BLK and 338 ARC ammunition, which are designed to work with a rifling-twist rate of 1:8-inch, the 8.6 BLK uses a rifling-twist rate of 1:3-inch. This is an incredibly fast twist rate, but what’s it for? Well, it helps better stabilize bullets that are about the size of your little finger, but it also significantly enhances terminal performance.”