Trouble getting bullets to stop at solid blocks

I'm currently working on my first game (a 3D side scroller. It's a parody of Super Mario Bros) and until now I've been able to find answers via google searches, but now I've stumbled across a problem...

To avoid any nasty pushing-around physics, the bullets in my game (shirukens or fireballs, currently just shirukens) have a box collider and is set to a trigger, but no character controller. To move, it uses transform.position. Anyway, it collides with enemies and other triggers just fine, but when I try to hit a solid object (whether it be plane or cliff box), it doesn't process any of those collisions.

Code attached:

 #pragma strict
 
 private final var gravityRate : double = 25.0;
 private final var gravityMax : double = 100.0;
 private final var deathTimeMax : double = 0.5;
 private var gravity : double = 0.0;
 private var deathTime : double = 0.0;
 var direction : Vector3;
 var throwSound : AudioSource;
 var hitSound : AudioSource;
 
 
 function Start () {
     Debug.Log("Creating a g**d*** shiruken");
     throwSound.Play(0);
     
 }
 
 function Update () {
     if(deathTime > 0.0)
     {
         //Give the sound effect some time to work it's magic
         deathTime -= Time.deltaTime;
         
         if(deathTime <= 0.0)
         {
             deathTime = 0;
             Destroy(this.gameObject);
         }
         return;
     }
 
     
     gravity += gravityRate * Time.deltaTime;
     if(gravity > gravityMax)
         gravity = gravityMax;
         
     var offset : Vector3 = transform.position;
     offset.x += (direction.x * 30) * Time.deltaTime;
     offset.y += (direction.y * 15) * Time.deltaTime;
     offset.y -= gravity * Time.deltaTime;
     
     transform.position = offset;
     
 }
 
 function OnTriggerEnter(collision : Collider)
 {
     var other : GameObject = collision.gameObject;
     
     Debug.Log("OUCH!  " + other.tag);
     
     if(other.tag == "Enemy")
     {
         deathTime = deathTimeMax;
         hitSound.Play(0);
         var script : EnemyDamage = other.GetComponent(EnemyDamage);
         if(script != null)
             script.Damage();
     }
 }