see the laser designator tute in the ed depot for how to use the laser designator dot.
you can getpos a round, but you have to detect when it was fired using an event handler.
eg, if you did this in the player's init field:
this addeventhandler ["Fired",{ the_round = nearestObject[(_this select 0),(_this select 4)] }]
the global variable the_round would equal the round fired by the player.
you can make any vehicle fire blanks by using:
this addeventhandler ["Fired",{ deletevehicle nearestObject[(_this select 0),(_this select 4)] }]
in the init field