7 submarines are placed on an x, y grid - 0 to 111. They are located on whole numbered locations. Each submarine starts from any selected location with a given fixed speed and fixed direction, reaching another point on the grid every second. The submarines continue simultaneously in the given direction and speed thus reaching the border and deflecting by an elastic collision - same angel about the perpendicular to the edge. Assume no collision with a corner. More then one submarine may share the same location/grid-point - if a location includes more than one submarine all are destroyed.
You are armed with a unique gun that kills a submarine with its bullets. You are allowed to use the gun every second, when submarine reaches a grid point. The gun is not limited with its range, the accuracy is absolute, and you are armed with infinite bullets.
Devise a strategy by which you will kill all 7 submarines in a finite time. How long it will take at most?