As mentioned, you should find a "false" bottom under the ring cap. The false cap is not as air tight as the ring cap but once you insert it into the proper sized caulking gun it will be pretty air tight.
After trimming the tip you should have to run a nail into the tip to pierce yet another seal under the tip. Some caulk guns come with an attached wire for use in piercing.
Some people insert a nail to seal the tip after use, some have used a plastic wire nut, I found some soft rubber tips that make a pretty good seal over the tip. None of the methods seem to keep the material as soft as new for all that long.
Without adult supervision...
Quando omni flunkus, moritati.
I'm a man, but I can change, if I have to, I guess.