Raimey - There are three distinct sets of gas involved in a gun discharge. So far, I have been speaking of the gas evolving from the powder burn. It is hot enough that the speed of sound in it will be higher than we normally associate with gun discharge velocities. The second gas set is the air in the barrel in front of the wad. It will be compressed by the discharge, but much will flow out of the barrel prior to the shot exceeding its sonic velocity. The third set is the air outside the barrel. It will have typical sonic velocity.
The shot in the barrel will acclelrate to some velocity based on pressure of the powder gas on the wad base, friction of the wad/shot to barrel, and back pressure of the air in front of the wad. The powder gas can't flow at supersonic speed without passing through a nozzle, but doesn't need to as it can flow fast enough at subsonic (for it) speeds to achieve the needed shot velocities. The air in front of the shot is simply pushed out and we know that its compressive pressure is less than the pressure of the powder gas as shot loads continue to accelerate in long barrels. As the shot load exits into the ambient air, it may well be above the sonic speed of that air. Remember, sonic speed depends on temperature of the gas (air). As the shot charge exits the muzzle, a sonic shock wave will form on the bow of the charge and will trail away from it until the charge drops below sonic velocity. Muzzle velocities for shotguns typically range from 900 fps (slow) to 1500 fps (very fast). Typical sonic velocity of ambient air at the earth's surface is around 1100 fps.
How fast? Well, as I recall the limit for powder gas in a barrel is around 10,000 fps. If you need to go faster, you need a rocket or a rail gun.
Again, if that is not clear, ask questions - I should be able to explain this.
