i doubt you can get a vehicle to do that, but if it's a bot, most likely you can
input>self>setvehicle[tank turret]
onbotspawn>bot>setaim
onbotspawn>bot>spawnprojectile[tankshell]
onbotspawn>self>firerelay - delayed 30000
onrelay>self>setvehicle[none] - delayed 30000