The following works for me, but don't know if it will work with all hard capped cartos. I find a screw that's just small enough to fit into the hole in the cap, but not so small that it goes too deep. Screw it in just enough so it grabs tightly but not so far that it hits anything. Maybe about 1/16 - 1/8 inch? Grab the carto in one hand, the screw in the other, then pull really hard. Usually there's no damage to the cap, but occassionally it would get just a little bent. I think it's a #5 or #6 wood screw, medium thread pitch.