I'm not talking about flow or gains, I'm talking about a .750" maximum outside diameter pipe that ends up being very thin wall for something that see's ~1200 degrees with a tiny cross section and then we wonder why they break off or crack. Flow is relevant to INSIDE diameter and they need to allow a thicker wall tube that doesn't break and set a spec for a maximum inside diameter and call it good. 1/2" schedule 40 pipe has an nominal inside dimension of .620" +/- with almost 1/8" wall thickness. It's cheaper, easier to bend without kinking, and much stronger.