I will start that this is a suggestion not a demand, but i refrained from posting this in suggestion thread Since this is something both big and would require a lot of thought put into this.
First of all, this idea was born because there are, in my opinion, to few decent high tier weapons in the game, and conventional munition is treated only in realm of either early game muskets or later as cheap but short range PD.
I think that Railguns, (as shown by army at to this day) should be more viable especially in vacum of space and tonnage of fleet we are working with. Right now, rails systems are used only as low tier early weapon to play with natives. Here is a proposition of changes to make those from being bigger copy-paste of gauss'es to be something between particle lance and conventional torpedo:
01. Rail Cannon should negate shields, but do not negate armor, as all kinetic projectiles do.
02. Rail Cannon should have the same pattern as particle lance (not 100% sure if its like that now, never used too many railguns in ship-to-ship combat).
03. Since Rail Cannon could acquire the same energy as particle lance, so damage would be at least the same if not bigger. However devastating to the internals, damage should be dampened by 1/3 or 1/2 to armor both for physics and balance reasons. (composite and ablative armors are made to absorb the shock of kinetic projectiles, and that would fit railgun slug nature)
04. Rail Cannon should be single shot, with huge reload penalty (depending on size but starting at least with 60s for the smallest ones). In real life each shot damages barrel rail. I am not sure if that would be viable or possible but each shot would require some maintenance supplies.
05. Not sure if this is possible to code up, but unlike gauss cannons that uses maglev system, Railguns uses electric system to propel the slug, and thus should require very specific projectile (this has also potential to designate rounds, similar to rockets, in the future?), an thus also would requiring magazines (although without warheads it would not contribute to explosions) even if only 1ms per projectile, it would be counterbalance to the effectiveness. Also please not that difference between cannon and missile is that railgun require specific dimension of the projectile, not like in case launchers that can launch everything up to launcher size.
06. Range - This would mostly depend on difference between railgun size and projectiles size times speed. Now, since gauss system uses EM field to transfer energy, speed achieved is much lower, than rail system. I would propose initially speed of around 100 000 km/s, and since aurora uses 5s intervals it would calculate base accuracy based on that (100 000km/s * 5s = 500 000km for shot without accuracy penalty, 750kkm for 50% penalty, capped at 1mln km with no chances at all) Although i would love to imagine railguns as long-range kinetic weapons.
07. further counterbalance would require at least sizes like spinal mounts (800-1000t for the smallest ones), no turret mounts, and no usage in point-defence scenarios.
08. A lot of EM echo on each shot.
09. speed may be adjusted based on rail cannon size, launch velocity speed, projectile (mass, maybe composition in future?).
10. There is also potential to add new component - energy capacitor - as a separate entity from all the weapons - that would play nicely with all weapons for example having two capacitors would speed up energy weapon recovery, in exchange for additional mass. Also it would determine performance of the rail system (shooting from multiple capacitors, or only one, using capacitors form other weapons if those are out of range).
11. Reload penalty should be related to size of the cannon and mass of handled projectile.
12. and the last thing - great from pounding ground units from orbit.
please give it a thought