Converting to degrees, you would have to be holding the gun a horizontal polar rotation of 26.5 degrees leftwards in relation to the x axis, and vertical polar rotation 14 degrees upwards in relation to the x axis(plus or minus a few decimal points).
Of course this is using standard coordinates, but games usually run the y axis from top-to-bottom, rather than bottom-to-top, so you might have to hold the gun 26.5 degrees to the right, not the left.
It's called trigonometry people....
In 2d (ignoring z axis)
tan((4-2)/(8-4)) = theta
theta = 0.463647 Radians
In 2d (ignoring y axis)
tan((2-1)/(8-4)) = theta
theta = 0.2444978
Converting to degrees, you would have to be holding the gun a horizontal polar rotation of 26.5 degrees leftwards in relation to the x axis, and vertical polar rotation 14 degrees upwards in relation to the x axis(plus or minus a few decimal points).
Of course this is using standard coordinates, but games usually run the y axis from top-to-bottom, rather than bottom-to-top, so you might have to hold the gun 26.5 degrees to the right, not the left.