A wellhead seal assembly that forms a metal-to-metal seal between inner and outer wellhead members. A seal member has inner and outer walls separated by a slot, where the slot has an upper portion that is wider than a lower portion of the slot. An energizing ring having an upper end portion and a nose is moved into the slot, where the upper end portion has a greater cross-sectional thickness than the nose. As the energizing ring is moved into the slot, the nose of the energizing ring engages the lower portion of the slot to form a lock against the walls of the inner and outer wellhead members, and the upper end portion of the energizing ring engages the upper portion of the slot to form a seal against the walls of the inner and outer wellhead members.