Hi Shane,
  The difference with GMAW and FCAW is that with FCAW, each droplet is encased with the inner shielding. In other words, each droplet of the FCAW is protected by the slag from the inner shielding. Thus, protecting it from the carbon in the gas. GMAW shielding is dependent upon an inert gas for shielding, and carbon can be obsorbed in the weld pool if the carbon content of the gas is high enough. Please let me make one thing clear..the higher the heat input during GMAW welding, the higher propensity for carbon pick-up from the shielding gas. That is why FCAW is used with either straight CO2 or 75/25. To be honest, ANSI/AWS A5.22, Table 2, subsection c. states that any other medium that the electrodes are found to be suitable can be used. As far as straight CO2 and 75/25, those are the gases that are used to test the FCAW at the factory for classification tests only. That does not mean that no other gases are allowed, just what was used to classify the FCAW. Tests have shown that 75/25 gives better weld bead appearance and weldability than straight CO2, but either will work. In GMAW, anything over 3-5% CO2 should be carefully considered when using GMAW. Of course, in the spray mode of transfer there is a greater chance of carbon pick-up than in short-arc mode. One last thing, whether the FCAW is a -1 or -4, either can be used with straight CO2 or 75/25. The -1 and -4 only designates what was used at the factory to classify that particular electrode. Nothing more...

Chuck