Owner

The "owner" of the sprite. If the sprite was spawned by another sprite, then owner holds the ID number of that sprite. This also determines which player in multiplayer is awarded a frag if damage from an actor kills another player.

When a projectile is deflected by a mirror, it seems to take the ID from a random player (cooperative start positions, tag 1). That is why you don't get affected by your Shrinker blast radius, but can shrink yourself using a mirror...