A search for "Dual Shield" come up with this
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"The third kind of wire is Gas-Shielded Flux-Core, usually E71T-1.
This is called Outer-Shield by Lincoln and Dual-Shield by ESAB.
This wire combines the properties of the previous two wire types.
It uses a Flux-Core wire and a gas shield, hence "dual" shield.
This kind of wire is the best for welding heavy steel in a shop.
The gas shield restricts it to indoor use, but it welds very heavy steel in a single pass with little or no spatter and near perfect welds.
It rules the industry in heavy shop fabrication.
You still have some smoke and the slag layer to clean up, but absolutely lovely welds.
Dual-Shield requires very high amperages and requires a heavy duty machine and a tougher gun to handle the amperage involved.
The wires that use a gas shield can be used with pure CO2 or Argon/CO2 mixes.
Pure CO2 is very hot and very cheap, but it does make a lot of spatter, which makes for a lot of clean up afterwards
Argon added to CO2 makes the arc smoother and cleaner.
The most common gas mix for steel MIG is C25, which means 75%
argon/25% CO2"
from here
http://en.allexperts.com/q/Welding-3487/Shielding-Bottle-MIG-Welder.htm
Thanks for the clue...