This might sound like a scrub problem but just in case you run into the same problem I did here’s a little tip:
2D and 3D box colliders don’t interact with each other.
I was having the damnedest time trying to figure out why my projectiles weren’t being shredded and it turned out that the projectiles had 2D box colliders and the shredder had a 3D box collider. It wasn’t until I changed the shredder’s collider to 2D that the projectiles started getting destroyed.