If you fire a shell against the supposed spin of the earth then surely you would have to apply force to overcome inertia ? Can't see any column in the fire chart allowing for direction of fire v spin direction of earth (or rotation v latitude ) .
No. You’re misunderstanding.
Get on a constant velocity train, bus, or airplane, stand in the middle with one person on each end. Throw a rock as hard as you can at person A standing at the front of the car then throw a rock at person B standing at the back of the car.
You will hit both people equally hard and potentially injure them. They are on your train. They are moving the same speed you are. Inertia.
Now if you want to throw a rock at a pedestrian on the sidewalk from your train after you pass them, THEN you would have to overcome your inertia if you wanted to hit them with that same amount of force.
Or if you wanted to shoot a spaceship that is at rest in relation to the earth.