FYI in case it matters I was doing this all in Blueprints.
I had the exact same problem. I had the projectile velocity set to the forward vector of the arrow. I fixed it by just setting the projectiles x-axis velocity to the speed I wanted it to travel at. I’m guessing projectile velocity is relative to the projectile and by using the forward vector it added velocity to the projectile’s other axi even though I just wanted it to shoot in the direction I was aiming. And I’m guessing you’re doing the same thing too in line 22. Try taking out ShootDirection from line 22 maybe (or maybe line12). Sorry I’m not a programmer.