There are no metal carts
Cartridges aka carts are plastic are push over or into the atomizer of a 3-piece ecig. But carts are annoying - they need
juice added very often and they can dump the
juice in your mouth. And atomizers can die and need to be kept clean. The most popular 3-piece ecigs also have blank cartomizers available at hardware vendors and then you can use them as a 2-piece.
Cartomizers aka cartos are preferred by most people over carts, and they are mostly metal and screw right on the battery. A cart can be reused over and over and cleaned and restuffed used more. Good ecig models have many types of blank cartomizers available from many vendors.
510 and KR808D-1 are the two most supported models for cartomizers. 510 is most supported but there are few prefilled cartos. KR808D-1 (which is also Vapor4Life.com) has more prefilled ones but still plenty of blank types. Cartomziers have limited lives and are considered disposable. A cartomizer, if kept damp with
juice, may last from 4-14 days depending on the type of carto and how heavily it is used. Some people are successful with washing, drying and getting more use out of a carto after that but don't count on it.
And I strongly recommend you get manual batteries. Both 510 and KR808D-1 are available with manual batteries. Autos are more likely to be killed with juice leaks and autos can accidentally turn on from wind or vibration. Manuals give you better control of your hit.