There would be an incredibly loud sonic boom when the projectile left the unevacuated barrel, but since the projectile is going faster than the speed of sound,

There would be an incredibly loud sonic boom when the projectile left the unevacuated barrel, but since the projectile is going faster than the speed of sound, it would not benefit from the pressure wave; in fact, it would probably slow down, and possibly be destabilized when it penetrated the thick layer of (possibly plasma-phase) air. It is also likely that when the pressure wave left the barrel, the entire barrel would shake itself to pieces (unless it had some way of damping the axial waves going back and forth through the barrel, akin to a 'water-hammer' effect).