How thick of metal needs to be used for what applications?

I've been playing around with my little MiG, and can seem to get decent penetration on anything less than 1/8"(not sure if it's actually 1/8" or 11ga, I just used a tape). I figure that if I can get at both sides, I could do up to 3/16".

So based on this, what could I actually build myself? Are 12ga tubes and 11ga plate actually thick enough to be able to build anything useful? I'd like to be able to build myself a set of sliders, and maybe a bumper. But if I would just be wasting my time/money, I'll focus on small projects till I can pony up for a 220V welder(and the garage to go with it)

 

My bumpers and sliders are built with .120 wall tubing and 1/4" steel plate brackets. 11 and 12 gauge are plenty thick for tube work.

 

I have been welding 1 1/2" ID .150 wall black pipe with my little millermatic 135 Using solid wire with 75/25 gas shielding and it has done pretty well. I think it puts out 90-100 amps without wanting to shut off. I made my flatbed with it, and everything has held together well.

 

my sliders and the tubes on my bumper are all .120, and i have some 16ga, and some 3/16 on there too, but the winch part is all .25.