Ok, first of all you need a weapon. I'll post a few so that you havee better weapons like ak47s and desert eagles. Then your good to go. to add the object to the world, just click 'file','import', then select the object. NOTE: you may do this with worlds.
Ok, now make a method called 'Shoot', in that method you will put, (example)
'AK47 move backward 0.1 meters duration 0.01 seconds
AK47 move foward 0.1 meters duration 0.01 seconds'
Its backward deppending on the way the object was setup. If the direction is wrong, test different combonations of directions.