Adamantium armor is our full effort to create a material that isn't really all here. Adamantium armor, once formed, cannot be altered. It takes frightening amounts of energy to create from its natural elements. Once created, it holds onto its existence through an energy field that cycles through several different dimensions.
When Adamantium armor is struck by mass driver weapons, most of that mass driver shot gets teleported into several different dimensions. Only a fraction of the original mass is left to expend its kinetic energy against our ships.