So the idea goes like this: PDC's have fire rate and can shoot down missiles, but are rubbish at medium to long ranges due to the relatively low velocity and mass of the rounds. Missiles are mainstay anti-ship weaponry regardless of the ship size due to the adaptability of the payload, the ability to maneuver and make course corrections, but due to the time needed to accelerate to full velocity lest they get tracked and shot down easily, they are relegated to long ranges.
Meanwhile, railguns, capital-grade railguns specifically, fire rounds at a significant fraction of the speed of light. Such velocity brings sufficient kinetic energy to perforate multiple decks and rooms in a ship, dealing significant damage. However, the energy required to fire such a gun is also immense, requiring low rates of fire, else the mass driver overheats and damages itself. The railgun round also shares the same constraints as PDC's: the finite velocity allows for non-zero reaction time of the target to evade. This reaction time can be roughly estimated as the distance divided by the round velocity; thus, the railgun's range is limited at the top due to the time such distances give for evasion, and also limited at the bottom due to PDC dominance at close range, thanks to their high fire rate and sufficient damage output.
A railgun's range can be increased in battle through clever strategy, mind games, and prediction, theoretically to infinity, depending on the wits of the operator and the stupidity of the target. Of course, increasing the velocity also decreases reaction time, but given the first law of thermodynamics, and that KE is proportional to velocity squared, to decrease the reaction time (and increase the range) by a factor of 2, the energy input must be scaled up by a factor of four, dragging material requirements and thermal management with it as well. The improvement of the railgun's range technologically is one of diminishing returns. At some point a capital ship will not have the power generation and thermal management systems required to support a high velocity railgun, relegating such powerful weapons to stationary platforms.
Meanwhile, railguns, capital-grade railguns specifically, fire rounds at a significant fraction of the speed of light. Such velocity brings sufficient kinetic energy to perforate multiple decks and rooms in a ship, dealing significant damage. However, the energy required to fire such a gun is also immense, requiring low rates of fire, else the mass driver overheats and damages itself. The railgun round also shares the same constraints as PDC's: the finite velocity allows for non-zero reaction time of the target to evade. This reaction time can be roughly estimated as the distance divided by the round velocity; thus, the railgun's range is limited at the top due to the time such distances give for evasion, and also limited at the bottom due to PDC dominance at close range, thanks to their high fire rate and sufficient damage output.
A railgun's range can be increased in battle through clever strategy, mind games, and prediction, theoretically to infinity, depending on the wits of the operator and the stupidity of the target. Of course, increasing the velocity also decreases reaction time, but given the first law of thermodynamics, and that KE is proportional to velocity squared, to decrease the reaction time (and increase the range) by a factor of 2, the energy input must be scaled up by a factor of four, dragging material requirements and thermal management with it as well. The improvement of the railgun's range technologically is one of diminishing returns. At some point a capital ship will not have the power generation and thermal management systems required to support a high velocity railgun, relegating such powerful weapons to stationary platforms.