Make collider work for some objects

OK, right know I want to create a dropshield like halo reach game. As you know the dropshield not permit the pass of bullets, grenades, rockets, etc., but the player can get in or get out of the dropshield, that means that it has a collider that permit only the pass of a player. I know it could be a simple solution if you make it trigger and when the bullet collides(OnTriggerEnter)the bullet is destroy, but I actually want to make it bounce and also with grenades wont work. So I want a know if there is a code that permit this or a tactic to make the dropshield. Basically I want to make something like this:


Set your shield and player to different layers, then use the Layer Collision Matrix in Edit > Project Settings > Physics to alter what collides with other things.