You just gotta love the TML. A post seeking help in modeling the Galactica's main guns morphs into a discussion of (essentially) "near-C rocks."

I still think a tapered-bore autocannon of sufficient mass and with a barrel of artificial diamond best matches the pictures and videos of Galactica's guns. If you look at them closely, the quite short barrels step down three (or even possibly four) times in external diameter between turret casing and muzzle.

As to propulsion method, a primary chemical or possibly electrochemical propellant with a secondary electromagnetic boost/guidance (to justify the triple rails forming a cage around the exterior of each main gun barrel) seems reasonable. Another reason I like a primary chemical/electrochemical propellant is the concern we see folks expressing about ammo resupply. Manufacturing simple iron bars would be a lot easier (and thus less anxiety producing) than creating chemical propellants and the consequently much-more-complicated shells to contain these.