Just to be fair, while this experiment as performed cannot prove flat or round Earth, it does substantiate three hard facts.

Number one is that gravity works under the given conditions.

Number two is that idiots will gleefully stand on a flatish piece of sphere under a six-hundred sixty grain projectile falling randomly towards them at about two-hundred miles per hour.

Number three is that they will not die from brain injury but rather from damage at or below neck level.

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: This experiment can't fly.

: Air and wind are the confounding factors.

: If this operation could be performed in a vacuum it would
: prove that the Earth is a sphere.

: The rifle has a rotational velocity with respect to the Earth

: at a given radius from the center of the Earth. The
: projectile has that same quality until it clears the bore
: because the bore carries the round.

: At dead stop the projectile has a longer radius than the rifle
: but the same rotational velocity with respect to Earth. It
: cannot complete the same degree of arc as the rifle and it
: would land in the line of rotation and slightly behind the
: rifle. If all things were precise additional rounds would
: land on top of each other.

: If the Earth were flat all rounds would fall right back down
: the bore.