Frosty, thanks for the reminder that the refractory liner needs to be included, that dropped my volume about 90 ci.
Now admittedly this is my first propane build, and I was double checking numbers trying to get a good orifice size, right now I'm at .030 mig tip, and found this and the post that is previously linked that had lots of Ron Reil info.
I also came across this page, http://www.joppaglass.com/burner/highp_chart.html. So trying to figure how much btu I might need, I realized that it really depends on the mass of steel I am trying to weld.
Just for example, and prob not 100% accurate, say it takes 100 btu to boil a pint of water in 5 min, works well and heats up my ramen noodles. But that same 100 btu will not the 6 quarts of water for my pasta noodles, it may get there given enough time and insulation, but I'm not waiting that long.
That all said if I can't get to the amount of heat (btu's) needed I could rebuild with another burner; move up to a larger orifice, until I run into the limits of airflow in an atmospheric burner; increase gas pressure, with the same airflow restrictions.