To under stand what I'm getting at, let me preface this with relativity. As when a bullet is being hurled at a ship, from the bullets perspective, the ship is being hurled at the bullet.
So, what does this mean? If the collision has enough energy, both the bullet and the object it hits gets annihilated. The same thing will happen if the bullet hits PD bullet or a piece of flack. In effect, the faster the bullet is traveling, the more kinetic energy it has. the less likely it would be able to survive and intercepting bullet. the less kinetic energy the rail gun bullet has, the less chance the bullet has of annihilating.
What this will do? massive spinal mounted rail guns will more eastly be counted by PD and flack, while shorter rail guns are less counted by PD and flack. giving the decision to build shorter vs longer rail guns more depth.
If we wanna play with this idea more. One of the things we could do is have the bullet shatter instead of annihilate. Turning into a shotgun like projectile at the point of interception. The fragments should have lower penetration. And the more kinetic energy the rail gun bullet had, the higher the spread. But the more fragments and/or the more damage the fragments will do.