I'm not quite sure what the effect is you want..
Right now you are changing the direction of the bullet 180 degrees immediately on collision.
It's not really strange that this produces a "corner", going from 400px per second Left to going to 400px per second Right
There are many ways to "solve" this, depending on your needs.
Decreasing the speed depending on the distance to the turning-point would be the easiest way while keeping the rest of your setup nearly the same,
but there are probably much better ways of achieving this (using different behaviours and/or without behaviours) if it was clear what you are trying to achieve and when and why..