I have often had problems with tubes of silicon calk, no-nails etc. Then recently I aquired some stoppers that screw onto the nozzles supplied with the tubes of gunge. BUT once I had withdrawn the pressure applied by the "gun" I unscrewed the nozzle, wrapped a piece of cling-wrap over the exposed top, screwed the nozzle down and screwed down the stopper.
When I needed the tube sometime later, to my surprise it was all fresh. I had expected to at least have to use a fresh nozzle but I didn't have to.
HTH, Alan