OK, i think you are looking at the problem a little too simply. It is
not just a relative motion issue. Air travelling towards the aircraft
would not be able to enter the muzzle (as it has nowhere to go). The
bullet would then travel down the muzzle at the speed of sound say (I
know it has to accelerate but lets just say it travels at a constant
speed) relative to the muzzle (i.e. the bullet would be travelling its
velocity plus that of concorde). Upon reaching the end of the muzzle
it would hit a wall of air travelling at the same speed as it so would
effectively instantaneously slow down to the speed of concorde. It
then could go one of 3 ways, it could bobble about in the space between
the end of the muzzle and the air around (the boundary layer), fly back
past the aircraft and possibly hitting the aircraft on the way past or
it could go bounce back down the muzzle.