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I've done the experiment with a gun--sort of--to convince a
companion. Pellet rifle shot over a calm lake, another pellet dropped
vertically. The splashes were, at least, very close in time.
For a bullet fired in a vacuum, so air resistance doesn't matter, one
can easily calculate the accuracy required in holding the rifle level
to achieve an equal time result within ten percent with fired and
dropped bullets. The result is, for a bullet with a muzzle velocity
of the speed of sound* and a height of 1.5 m above the lake level,
the rifle must be held horizontal to better than two milliradians,
and the requirement for accuracy increases with increasing muzzle
velocity. It is certainly possible to hold on target to this accuracy,
even offhand. If one has a lake that is a kilometer or more from bank
to bank then the feat could be accomplished by aiming at the waterline
on the opposite bank, or better still at a point approximately 1.5
meter above it, for any reasonably wide lake (even 100 meters) with
the rifle boresighted.
Don't speculate; calculate! Real numbers may prove enlightening.
Leigh