Baloney
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Talk to some expert artillerymen or folks at NASA charged with computing the trajectories of rocket launches.
We’re talking man-portable rifles here, not rockets or other things traveling more than dozens of miles in seconds.
Does it exist? Yes. Does it matter for sub-1” projectiles? No.
The longer a projectile, any projectile, is in the air, the more likely planetary rotation comes into play. Field Artillerymen and mortar gunners understand this with “high angle” fire, where you are positioning your firing tube at angles above 45 degrees (800 mils) elevation. (usually to fire over an obstacle, such as a hill or mountain.
As time of flight increases, the more the movement of the Earth during flight impacts where your round will strike. Non-guided weapons have to aim where the target will be, so manufacturers provide tables that allow fire direction centers to account for planetary movement during flight over given ranges and times of flight. Additionally, we also have to account for wind speed, temperture, air pressure, target elevation differences, and other factors to hit something from far away.
The effect is real, and has been understood for decades by those of us who throw projectiles through the air for a living.
I wouldn't discount the Coriolis Effect on rocket launches or other, military-grade ballistic weapons. It's just not something I would consider for any sort of civilian weapon I would consider firing.