There are a number of cannons that never get to meet the line that the reticle crosshair draws between the gun’s barrel and the horizon (which in most cases coincides with the aircraft’s trajectory). Some examples are:
-Type 99 japanese naval 20mm cannons
Notice how the rounds never coincide with the plane’s trajectory marked by the tiny round circle over the crosshair
At least in props, most gunsights are calibrated for ~300 or 400 meter convergence & vertical targetting. Shooting without VT means the bullet wont behave as the gunsight advises.