Concorde Vs Bullet

If concorde (or and other supersonic
aircraft for that matter) was fitted with guns then the bullets would
not be able to leave the muzzle

I don't think there's anything special about being supersonic to the
basic physics involved (though it will add some wrinkles). Before
firing, there will be ram air pressure in the (forward facing) muzzle.
When the bullet is fired, there will be explosive chemical pressure
behind the bullet which will exceed the ram air pressure in the muzzle -
this difference will cause the bullet to accelerate down the muzzle. At
some point the pressure behind the bullet will begin to decrease (as the
reaction ends and the volume continues to increase), while the ram air
pressure will increase further up the muzzle (due to the bullet pushing
the air away). Given a sufficient charge, the bullet will exit the
muzzle and become a projectile. The speed of exit depends on the size
of the charge - obviously a dud would not eject the bullet (but this is
true of anything).

Once the bullet exits the muzzle, it will slow down and descend at a
rate typical of a bullet. The plane flying behind or next to it is
independent and largely irrelevant, though it will maintain speed and
altitude due to its engines.

Bullets typically are supersonic to begin with, so there's nothing
special here. Coming out of the concorde it will have an initially
higher airspeed than a typical bullet shot from the ground, so it will
decelerate more quickly. My gut feeling (I have never shot a gun
however) is that the bullet will have enough excess forward speed that
by the time it slows down to the concorde's speed, it will be way ahead
of the plane.

Do fighter planes have guns that fire at supersonic speeds? I suspect so.

Jose
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