Spaceships: Armaments and Wrecking Balls
The smallest railgun spinal mount is available at technological level thirteen, though at twenty eight hundred tonnes, small is a near relativistic term, much like the speed of the bowling balls it spits out.
There are number of reasons that you can't use this as a planetary defence weapon, primarily because it's limited to medium range, which means that any attacker is going to sit put and just flatten it.
Assuming there are other targets more worthy of their attention, and the attackers start pressing closer into range, the next issue would be gravity; what goes up and doesn't escape, is gong to come back down. An atmosphere should also slow down that ball bearing, and should create quite a spectacular fireball, maybe a Tunguska style blast in the surrounding environs, considering how fast it will be going.
So what's it good for, once particle accelerators and meson guns become available?
That depends on whether you can apply other advantages to the weapon system, besides cost and size reductions.
The obvious one is long range, available at technological level twelve, and resilient at thirteen. Juice is less of an issue, since five hundred scotts per size increment is thirty four tonnes of basic fusion plant, and half the requirement of an equivalent particle accelerator, though surprisingly the same as a meson gun, and at half the price.
A modern, well rounded light cruiser would be using the particle accelerator equivalent, but one that designed more for the commerce protection role, or cruising around the outposts of the empire, actual colonial cruisers, not the Kinunir, where the implied threat of bombardment or gunboat diplomacy is more effective than actual infliction of it, having a cheap, easily maintained spinal mount in orbit, or chasing off the better armed commerce raiders is an attractive proposition. Unlike missiles and power plants, wrecking balls don't deteriorate or require maintenance.
