Can a rifle fire a bullet faster than the average molecular speed of the ges pressurizing the bullet?

Gas pressure is generated by the movement of molecules. Can a bullet propelled by a gas accelerate to a speed much higher than the average molecular speed? I would think the gas simply cannot expand fast enough, no matter how hot and pressurized it is, but in NASA they have a gun capable of accelerating projectiles up to 6 km/s, 20 times the speed of sound. How is that possible?