You can't dual shield MIG (GMAW) wire to my knowledge. Could you be talking about Flux Cored (FCAW) perhaps? Flux Cored can be dual shielded, but I agree it's definitely not the right process for the bike frame. FCAW is designed for thicker metals, burns hotter and has more penetration, not to mention you have to deal with the slag.

I'm using solid wire MIG (GMAW) with C25, designed for thinner metals. I've welded 16ga with no issues, and the welder will do down to 24ga. From my research on MIG vs TIG when welding 4130 tubing (mostly on the auto racing forums), TIG is indeed better if you are a very skilled TIG welder, but MIG is more than capable. I've seen s few builders out there using MIG for their frames, there are even some MIGged frames at Target, though most are TIGged. In my case, the MIG welder I have is infinitely better than the TIG welder I don't.

Here's a guide on the miller web site about MIG welding 4130. Scroll down a bit.

http://www.millerwelds.com/powerclic...lick_0307.html

Thanks everyone for your feedback. Still reading the links you provided Rodar, and thanks for the tip on Dillsburg unter. The HT info was useful as well.