No weapon design system for T20 but Traveller's Aide #6 Against Gravity has among other vehicle weapons mass driver cannons ranging from 35mm - 18cm as well as a good mix of vehicles (including an artillery grav sled IIRC).
There is a weapon design section in the Mecha d20 SRD but I'm not sure how useful it would be. I've only used it to do some comparison tests with unarmed designs.
Casey
There is a weapon design section in the Mecha d20 SRD but I'm not sure how useful it would be. I've only used it to do some comparison tests with unarmed designs.
Casey