You have to be careful of the same system being present on capital ships. If you have a weapon that does, say, 25% of the damage of a single-shot 10cm railgun (49 tons) and is 25% of the size, then you actually have a problem where the smaller version is superior due to increased HTK slots, albeit this is a relatively minor advantage. This is why Steve set up the reduced-shot railguns to be slightly less tonnage-efficient than the four-shot versions. Therefore you will want to have some addition to ensure that the smaller weapon does not displace the larger weapon on large ships.
There should also be a reasonable tech progression to feel in line with the other weapon types. Given this I think probably the best (and simplest) implementation would be to extend the low range of the Gauss Cannon to perhaps 0.25 HS (12.5 tons) with 4% base accuracy - you can actually do this pretty trivially with a single DB edit if you want to try it in your own games by adding a line to the FCT_TechSystem table. This way, the "autocannons" benefit from improvements in Gauss ROF (and range, not that anyone cares) and the existing balance between components is unaffected. I also note that this way no new ammo consumption mechanic is needed which I would consider a good thing as I would not want the added micromanagement, as missiles are quite enough to keep track of in my humble opinion.
