Flux core is a type of MIG welding.... please kind of know what your talking about before you ask a question.... Gas assisted solid mig wire welding is neater and cleaner.
There are two types of flux core mig welding that you are talking about.
1. Gasless flux core mig wire 2. Shielded flux cored welding wire, uses a shielding gas as well.
Most likely you are talking about the gasless mig wire.
look here for more info:
www.learn-how-to-weld.com/gasless-mig-wire.html