Minecraft wiki entry for Projectile, associated text:  HasBeenShot: 1 or 0 (true/false) - true after this projectile exists for at least one tick. This is used by the game to ensure that it only triggers the projectile_shoot game event once.
 LeftOwner: 1 or 0 (true/false) - true if this projectile has escaped its owner's hitbox for the first time and can now collide with entities. Not present when false.
 Owner: The UUID of the entity this projectile was thrown by, stored as four ints. May not exist.