[Raging Deadstar]

If I recall correctly in the Mini Series 2 parter the Cylon Basestar attacked with nuclear Missiles, or some sort of missile based weapon.

The Cannons seemed to be for shooting down Fighters and Incoming Missiles. It makes sense that the Galactica would just weather it. Either its Military Capital Ship Ordinance was stripped when it was about to be decommissioned, or perhaps they know they have a limited amount and can't use them frivolously.