Almost definitely has to do with the way you’re setting the BodyVelocity. That would certainly cause a curve, as would using the incorrect relativity for the bullet. Try revising how you create it or look into VecrorForce.
By the way, stop using the parent argument of Instance.new. You’re registering items into the DataModel before they’ve been fully initialised with all the other property sets so this code is less performant to run. You should be running in this order:
Create part, set part properties, create attachments, set attachment properties, parent attachments, create trail, set trail properties, parent trail, create BodyVelocity, parent BodyVelocity, parent part.