A stopper for a medicament delivery device, the stopper comprising: a sealing base for sealing an opening of the device, the sealing base having a central axis; a plurality of elongated members extending away from the sealing base in a direction parallel to the central axis, the elongated members set radially away from the central axis and defining: a portion of a central well coaxial with the central axis; and, between circumferentially adjacent elongated members, a void defined by adjacent surfaces of the elongated members that are on the exterior of the stopper and that merge; wherein the stopper is configured to: in a first operational configuration, have the elongated members engage an inner wall of the device to support the sealing base away from the opening of the device such that the void provides exchange of gas between an interior and an exterior of the device; and, in a second operational configuration, have the sealing base engage the inner wall to seal the opening of the device; wherein the stopper further comprises a resilient material; wherein the resilient material is penetrable by a needle and self-sealing around an outer wall of the needle; wherein, in the second operational configuration, the needle extends from the exterior of the device through the sealing base into the central well; wherein the central well continuously tapers toward the sealing base; wherein adjacent surfaces of the elongated members that are interior to the stopper merge to form part of a webbing surface of the central well. a sealing base for sealing an opening of the device, the sealing base having a central axis; a plurality of elongated members extending away from the sealing base in a direction parallel to the central axis, the elongated members set radially away from the central axis and defining: a portion of a central well coaxial with the central axis; and, between circumferentially adjacent elongated members, a void defined by adjacent surfaces of the elongated members that are on the exterior of the stopper and that merge; a portion of a central well coaxial with the central axis; and, between circumferentially adjacent elongated members, a void defined by adjacent surfaces of the elongated members that are on the exterior of the stopper and that merge; in a first operational configuration, have the elongated members engage an inner wall of the device to support the sealing base away from the opening of the device such that the void provides exchange of gas between an interior and an exterior of the device; and, in a second operational configuration, have the sealing base engage the inner wall to seal the opening of the device; wherein the stopper further comprises a resilient material; wherein the resilient material is penetrable by a needle and self-sealing around an outer wall of the needle; wherein, in the second operational configuration, the needle extends from the exterior of the device through the sealing base into the central well; wherein the central well continuously tapers toward the sealing base; wherein adjacent surfaces of the elongated members that are interior to the stopper merge to form part of a webbing surface of the central well.