So is the projectile damaging the character who fires the weapon then destroying itself?
If that’s the problem, add a branch before casting to bp_ damagable. and check if OtherActor from the overlap event = Instigator. If it’s not, move on to the cast and stuff. Also, in the function which spawns the projectile, plug a reference to the charcter who owns the weapon into the instigator pin on the Spawn Actor node so that check will work.