I am 99% sure that a few years ago in Phoenix, there was serious or
fatal injury from a celebratory near vertical gun shot. As I recall,
the bullet entered the top of the skull. Since then there has been a
media campaign to warn people not to do this and the police have
attempted to clamp down. Thus it is known experimentally that
vertically fired bullets can be deadly.
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From Michael D. Edmiston, Ph.D.
(a) At what elevation must a typical gun be fired such that air friction
reduces the velocity sufficiently that the landing velocity is near
free-fall terminal velocity? I think we are agreed that a bullet fired
straight up will land at terminal velocity. I think we are also agreed
that a bullet fired at steep angle, but not straight up, will land with
some horizontal velocity in addition to terminal vertical velocity. Is
there an angle of launch above which a typical bullet is not lethal.
(And I think we realize this is gun and bullet dependent.)
I am no so sure that one can decompose air friction into components
like that. I think the horizontal velocity is going to effect the
vertical terminal velocity. The bullet/ air interaction is more
turbulent than viscous in nature so the drag force is proportional to
v^2 not v.