Fired projectile collision with player issue

Personally I'd use layers and just set the layer when you instantiate the prefab for you projectile.

Alternatively you could have a script on your projectile that would identify who fired it, then in your OnCollisionEnter() you could get this script from the projectile and see who's 'bullet' it is.

i.e.

 void OnCollisionEnter(Collision col)
 {
     MyBulletInfo bullet = col.GameObject.GetComponent<MyBullletInfo>();

     if(bullet == null) return;

     // does it match?
     if(bullet.Source == this.Source)
     {    // handle the hit
 
     }
 }

Where Source is some type that would identify the player/npc, i.e.:

 public enum ObjectOwner
 {
     Player,
     Enemy
 }

Naturally this could just be a bool, or alternatively, you could make Source an ID and give players and enemies a unique ID (i.e. a unique int) and then bullets would not hit shields they were aligned with, but could hit other shields, including enemy shields.

But I imagine somewhere you are doing:

 GameObject bullet = GameObject.Instantiate(MyActiveBullet) as GameObject;

In which case you surely can add another line:

 bullet.layer = wasFiredByPlayer ? LayerConstants.PlayerBullets : LayerConstants.EnemyBullets;

Or similar. This is much more efficient and there is no specialisation of your missile prefabs, you just need to know whether the player or an enemy fired a weapon and I imagine you will know that.

Naturally, you can then adjust the physics settings to determine which bullets can collide which which layers.

Presumably, this is the approach you were considering, perhaps it is made easier seeing how easy it would be to place the 'bullets' on the appropriate layer at runtime?

Hope that helps!