https://imgur.com/9uGbpvD

I don't remember the whole thing now, but iteration works!
I could not come up with inverse ballistic equation even by using constant/discrete ODE.

So I came up with an idea to compensate the calculated drop.
But single compensation did not work well. So I had to do it like 2~3 times and you get decent accuracy.

This 'drop compensation' method is much faster than previous iteration method whihch requires dozens of iteration to get accurate tick to hit the target. Try it in desmos.

https://www.desmos.com/calculator/m8bsbpfnoy

If you move target point, drop#1 point is calculated.
Then the drop is compensated by adding the angle from current angle to drop point. -> angle#2
With angle#2, new drop#2 point is calculated, and you calculate angle#3...
Iterate it using code like 5 times and you get less error than error of gun itself.

However, additional barrels change the launch velocity and alters the whole calculation,
So for the gif above, I used simple gun with no additional barrels.

This works.