Stick a long screw or bolt (a couple inches or so) in the end first.
I also try to keep several, clean cartridge nozzles around. If the calk has set up in the nozzle, I have found I can unscrew the old one and sometimes (up to several months) the caulk in the tube is still good. You can also buy spare 3M nozzles but they're around $1-2 each.