I also think the muzzle flash is from the sabot.
One advantage mentioned in TFA is that there are no combustibles in the ship's magazine. When you can treat your ammo the same way you treat the canned peaches, your ship has an incredible advantage over traditional warships.
Other advantages are longer range, simplified sight picture of a moving target (at 5,000 mph a truck 100 miles away is not going to move very far down the road), and pyrophoric behavior when depleted uranium is used in the projectile (in addition to the kinetic energy, you have the explosive behavior of releasing a burning hot cloud of uranium dust at the point of impact).
This is a truly nasty weapon.