What is the techno babble reason for rail guns and gauss weaponry to be poo?
I get why meson, microwave and laser are poo at long ranges, they loose cohesion with distance. A rail and gauss shouldn't be loosing velocity, and the mathematical interception models shouldn't be to hard to do, especially if they can calculate them for missiles.
I don´t realy know, but my guess would be: Effective range, not range per se (they don´t loose damage over distance, just have shorter range than lasers)
Both, gauss and railguns use a "bullet" which moves at less than lightspeed, so hitting a target over several lightseconds is just so much harder. Missiles, on the other hand, have their on-board-guidance. They follow the target like our fire-and-forget missiles today (think Hellfire or Harpoon) so they can compensate for the targets manouver during the flight.
Say, you are shooting at a target two lightseconds (600.000km) out.
Your target is a spherical spaceship 100m in diameter.
Your target can accelerate at 100g.
If you are using a laser, it takes the beam 2 seconds to reach the target.
After those 2 seconds, your target can be anywhere in a "circle" with a radius of 2000m, so a single shot has a chance to hit of 1 to (2000m / 100m)^2 = 1/400
If a railgun "bullet" moves at 50% lightspeed, the chance is only 1/4th of this.
Hm, thinking of this, realisticly, we probably shouldn´t be able to hit anything at 1+ lightsecond without internal guidance