Let's pause the theorycrafting for a second and look at where we are coming from for some context.

Spinal mounted weapons, at their essence provide
* Increased range and damage
At the cost of
*All the normal penalties inherent in increasing range/damage
*Only being able to fit 1 on your ship.

What is important to note is that spinal mount weapons do not alter the rules. All their calculations are done in exactly the same manner as a normal hull mounted weapon. Rate of fire, damage, range, everything is calculated exactly as if they are a weapon of exactly their size, the only change is that the size limitation is increased by 25%/50%.

With lasers, increasing the focal lense size by 25% increases the base damage, which in turn increases both the range and the energy consumption (which could result in reduced rate of fire if capacitor tech does not keep up).

If we apply the same approach to railguns, the solution seems simply to increase the calibre by 25% in a similar vein. Base damage increases which again causes an increase in range and energy consumption, all following the same rules as hull mounts.

Is there any reason railguns were not originally allowed to be set as spinal mounts under these rules?

I don't necessarily see the need for a separate kinetic weapon that fires one shot since that's basically just a reflavoured laser. I do though agree that it would be cool and good since the totally unbiased truth is that kinetics are infinitely superior to lasers. Perhaps instead, add an option to the railgun design that lets us choose how many shots a railgun will fire per shot, and divide the damage accordingly?

Alternatively for a separate weapon, I really do like the idea of a custom-ammo flinging cannon. Perhaps allow us to design cannon shells with various effects (flak = lots of str-1 hits, penetrator = lower overall damage but only deals damage to a single armour column instead of spreading out, etc) and store these shells in magazines(?). The downside is that it might be more micromanagement, but its nothing that missiles don't already create. The obvious upside to this being a beam weapon and not just missiles is that the weapon can't be intercepted by point defence or AMMs.

What's the difference between a bomb-gun and an extremely short-ranged missile? Type of fire control needed?

If it uses beam FC, that would be very different  but would it have ammo though? Would we need a magazine, with bombs to use it?

I mean, if it were actually a bomb gun like mentioned earlier, that would be fairly unique.

It could make more damage but be shorter range, kinda like the plasma carronade, but good.

If you have a railgun that fires a single massive projectile don't you just (In Aurora terms) now have a gauss cannon?

Renegade Legion: Leviathan's crowbar hit or as I called it "Thunderstrike" was not "flak." It was closer to watching a good chunk of the enemy ship vaporize.  That is a spinal mounted mass driver in action.   A BB sized one (several km long) was basically instant death if it hit, so far as my memory goes.  I only ever got to fire one twice...one hit one miss...

Makes will power saving throw!