If I want to build a railgun, how do I calculate the length of the rails?
The force available to accelerate the projectile decreases over time and distance. At the same time, the deceleration forces are increasing (friction with the rails, air resistance, etc.) You need to find the crossover point where the net force drops to zero, and have the rails end somewhat before that point. That's where you'll have the greatest "muzzle velocity".