[quote=@FrostedCaramel]The conventional railgun does normally fire solid ultra-dense slugs. If there are correctable railguns slugs, then the possibility of a knife fight is essentially null. Who ever has the best railgun wins, hands down every time. Your plasma arc will do nothing to the slug other than turn it into a cloud of molten hot metal still traveling as fast as it was before, if the arc even has enough time to act upon it. CIWS will not be able to intercept something so fast and a missile would face the same interception problem. A ship could not out run a self correcting round, at least not larger ships. I find the idea to be a bit OP.
[/quote]Alright, since Keyguy haven't returned yet I think I can address this worry in better detail.
A missile has propulsion and overall it's kind of like a small automated spacecraft in capability. Now compare this with a guided projectile. Unlike missile which has on-board propulsion the projectile would only have some maneuvering thrusters for small course corrections and terminal guidance. They would not be as maneuverable as ships and their velocity would be realistically around 20-60 km/s plus the velocity of the launching ship. Even if we assume some fictional boost and call it 200km/s it'd be about the same velocity as the enemy spaceship going at average. It'd take about 1.5 hours to reach the target from 3 lightseconds while if you are lucky you can detect the projectile within 3-6 seconds. You have over an hour to react. Of course the projectile has sensors and can correct its path but it'd be a long chase. Said projectile has limited fuel and subpar maneuvering units compared to a full sized ship. The advantage over missiles would rather be cheapness, lightness and the fact it reaches the target from distance about twice as fast without less obvious signs of its arrival. Yes, this might make them better than missiles in a certain aspect but the latter has warheads and can use proximity detonation with a nuke or use even more standoff methods like nuke-pumped lasers or nuclear shaped charges. They also have more room for gadgets thus electronic warfare or even owning a CIWS is not out of question for these. So yeah, there's a benefit in both.

As for interception, aside from electronic warfare which is obviously plausible you can also shoot it down. CIWS can be of anything and if it's close enough then it cannot maneuver much so even coilguns with inferior speed can work. Plasma arc method is a bit iffy when it'd likely intercept from meters or perhaps just centimeters. But depending on intensity it works. Basically as the projectile is vaporized it'd turn on itself as an improvised ERA so the damage would be indeed less albeit not zero. Lasers would be a rather obvious choice and probably combat would involve finding means to overwhelm such defenses. And again, there is more than a hour of time before the projectile closes in. It's possible to launch your own guided projectiles or smaller missiles to intercept the target from long range.
Overall battles could be much more complex and exciting than just one side shooting their uber projectiles first and the enemy having no response against it. Much more.
This is also the part where submarine warfare comes in. Because anything but DEWs would be taking so long they're better compared to torpedoes.