You have to use up ammo until the target is hit - ten shots per round. Once he is hit, remaining shots are aimed at anyone in the line of fire. Use up any remaining rounds on a second target. If there's a third target and you still haven't fired all ten shots, keep shooting. As long as the target is in line with the intended target, you keep firing until you've used up ten shots.
Mortars fire high angle, and their shells land on top of objects.
They cannot fire directly, but must lob shells in an arc. A mortar has the same range as a cannon, but it cannot fire at targets less than R1 from it. Mortars can fire over walls, however. They cannot fire grapeshot. The third nearest the shooter is Close range, followed by Medium and ended by Long range.