Very good explanation, Unclenick.
But I would add that in the long run, you will be much better off with a separate crimping die. If you can't find one, buy a second seating die and remove the seating stem so that only the crimping function works.
The "expander/drop tube" should be a part of the "caliber kit" from Dillon. You should have a Dillon powder measure with it also. If not, it's well worth it to buy one.
Your 550 was designed for high speed reloading. Don't waste it's potential by using extra steps, like weighing powder charges. Get the accessories you need, if any, from Dillon and enjoy the speed built into your machine.
