Why isn’t there a checkbox under collision components for an actor (think Projectile) that allows them to ignore the specific actor that spawned them? It seems so simple, and SO VASTLY NECESSARY. Having your AI (or player) shoot itself is terribly annoying. This one little box would save HORDES of effort for the end user. Please consider it!
The only solution I found for this, and it wasn’t great, was to turn on and off my monster’s collision well it was in the process of throwing the fireball. I used the “Set Actor Enable Collision” and set it for Off when the throw starts, and On, when the throw concluded. In between there is a delay of 1 second. It’s very kludgy, but it worked.
I did try to spawn the fireball in front of the character, but that never worked. I really need my monsters to be able to shoot each other, as it’s quite funny, so immunity to all of their own projectiles wasn’t an option. I just didn’t want them to blow up the second they tried to throw a fireball, which is what they were doing. 
I recently found an EVEN BETTER way to fix this issue! I added a Billboard in front of the monster in question and use it for the location for the spawning. It’s always perfectly in front of the monster and nearly impossible for them to run into. It’s invisible and simple to add.