TonyB, I noticed no one answered your question about cartomizers. They are a combination of an atomizer & cartridge which will make a 510 2 piece. They hold from 20 to 30 drops of liquid & will last from 2 days to 2 weeks,then throw it away & get out another one.
I'm fairly new to the cartomizers(month) but have been very happy with them. A carto with the Ego/Tornado battery makes vaping a lot easier.
Good luck,
NB