Metal cores under AWS are considered as (C) composite wires under the solid wire specs such as A5.18.
Ultimately metal core, in my opinion, is its own beast. It is neither GMAW nor FCAW. But were I to decide I would have to go with AWS. There is no fluxing in metal core. No fast freezing, no mechanical enhancement, no deoxidation, no shielding gas formation. It is simply a powdered manifestation of GMAW to increase I2R heating, arc density, and therefore depostion rate.