When someone comes for the first time to the 1K range and tries to shoot something that won't stay supersonic at that distance (often a .30 caliber with the wrong bullet) we see great degradation of accuracy. In the pit, you know immediately if a round has gone sub-sonic on its journey to the target. When those go overhead, they end up all over the paper and off the target completely. Often they hit the berm, pissing everyone off in the pits.
Short answer, the bullets start to wobble, and once that happens they can go anywhere -- but, usually not where you want them to.