A delivery system assembly includes an elongate inner member that extends distally through a lumen of an elongate outer tube of the assembly; the outer tube is moveable relative to the inner member between first and second positions to deploy an implantable medical device that is held within the outer tube lumen. The inner member may include a flared distal end that abuts, and preferably conforms to, the proximal end of the device, when the device is held within the outer tube lumen. The assembly further includes a stability sheath that surrounds a limited length of the outer tube, in proximity to the handle, to provide an interface for both an operator, who handles the assembly, and for an introducer sheath that provides passage for the assembly into the venous system, so that movement of the outer tube is not hindered by either during device deployment.