Grammatico,
I am interested to find out too!
I am also interested in finding out how the tube is corrugated during its manufacture...in fact I want to know how the fire tube is sized, how are the corrugations and the thickness determined and everything there is to know about this HUSH! hush! subject...no place anybody describes the process...sounds like a proprietary thing.
Let me know what you find out and I will share what I find with you