Just to add to the other post,
Some (FCAW) fluxcore wires run straight CO2. We use ESAB 3/32" E70T-1 (flat/horz only) and 1/16" E71T-1 (all position) which are both dual shield 100%CO2 wires. We weld primarly all ASTM A36 or ASTM A572Gr50 (mild steel). Keep in mind, that fluxcore digs deep and will burn through on very thin materials (1/4" or less with the diameter wires we use). A lesser diameter wire will be needed on thinner materials. Some fluxcore wire are self-shielded and require no external gas coverage at all. Check the codes you are working to and see if they will allow self-shielded fluxcore. Some of our erectors use this type wire due to light wind and no need to lug a bottle up on the building.
John Wright