When using lerp it should be lerp(Self.BulletSpeed,600,1-0.1^dt)
change the 0.1 part to adjust speed of change , 0.001 for example will make it faster
And yes, as the post above says line 79 should be rotate 200*dt degrees and since you are using delta time instead of 200 it should be a much higher number, try 10000*dt or more and see what works best