An occlusive medical device system, comprising: an elongate shaft having a tubular body portion, a lumen extending longitudinally through the tubular body portion, and a plurality of retaining arms extending distally from the tubular body portion; an occlusive medical device configured to occlude fluid flow through a vessel lumen, the occlusive medical device including a proximal tubular mounting portion fixed to an expandable occlusive element; and a release wire disposed within the lumen of the tubular body portion and axially translatable between a distal engagement position and a proximal released position, the release wire having a distal engagement portion and a distal retention portion distal of the distal engagement portion; wherein the plurality of retaining arms extends into the proximal tubular mounting portion; wherein the distal engagement portion of the release wire is configured to engage the plurality of retaining arms in the distal engagement position, thereby urging the plurality of retaining arms radially outward into releasable engagement with the proximal tubular mounting portion of the occlusive medical device; wherein when the release wire is in the proximal released position, the plurality of retaining arms is deflectable radially inward to disengage from the proximal tubular mounting portion, wherein the distal retention portion includes a maximum outer extent greater than an inner diameter of the lumen in the distal engagement position, wherein the distal retention portion is non-linear, characterized in that the distal retention portion comprises a helical coil or a buckled tip. an elongate shaft having a tubular body portion, a lumen extending longitudinally through the tubular body portion, and a plurality of retaining arms extending distally from the tubular body portion; an occlusive medical device configured to occlude fluid flow through a vessel lumen, the occlusive medical device including a proximal tubular mounting portion fixed to an expandable occlusive element; and a release wire disposed within the lumen of the tubular body portion and axially translatable between a distal engagement position and a proximal released position, the release wire having a distal engagement portion and a distal retention portion distal of the distal engagement portion; wherein the plurality of retaining arms extends into the proximal tubular mounting portion; wherein the distal engagement portion of the release wire is configured to engage the plurality of retaining arms in the distal engagement position, thereby urging the plurality of retaining arms radially outward into releasable engagement with the proximal tubular mounting portion of the occlusive medical device; wherein when the release wire is in the proximal released position, the plurality of retaining arms is deflectable radially inward to disengage from the proximal tubular mounting portion, wherein the distal retention portion includes a maximum outer extent greater than an inner diameter of the lumen in the distal engagement position, wherein the distal retention portion is non-linear, characterized in that the distal retention portion comprises a helical coil or a buckled tip.