Depends to a certain extent what is specified by the wire manufacturer but in the UK the most common gas mix for welding carbon steel with cored wire is 80% argon/20% carbon dioxide and this is fine for solid wire on base material above 5mm thick or so.

Some cored wires run fine on a 100% CO2 mixture and you can get acceptable results with solid wire on carbon steel also but not as good as with a "mixed gas"

The only problems you would get would be if you were welding stainless steel, you would still be able to use 80/20 for the cored wired but would need to use a different gas (98/2) for solid wire

Hope this is of use

Cheers - Bob

Jer,

Always check the manufactures data.

Here are a few examples of metal core wire that require shielding gas common to hard wire.

http://www.hobartbrothers.com/pdf/MetalCor6.pdf

http://www.hobartbrothers.com/pdf/MetalCorEn_Vision.pdf

http://www.hobartbrothers.com/pdf/Metalloy70.pdf

FYI-

There is a good article in the January 2005 issue of "Practical Welding today" magazine titled:

"Simplifying Shielding Gas Selection- How different gases and gas blends affect your welding application".