My advice is set aside the steel rod and get the 40cr or 42cr hydraulic tubing listed in the ECM Tutorial document (16mm OD, 8.0 - 8.6mm ID). Then follow the tutorial. Yes, it’s more expensive than stainless steel rod. That’s because it’s actually designed to contain the kind of pressure a 9mm cartridge will produce. The 40cr and 42cr steel is significantly stronger and harder than your stainless steel rod (which is probably 304 or 316 alloy).

I can’t emphasize this enough – taking shortcuts when making a 9mm barrel puts you at high risk of serious injury or even death. If you can’t do it right due to cost or material availability, don’t do it.

i have no advice but just wanted to say all you that DIY the whole FGC-9 are on a dif level and i respect tf out of your dedication.

You’re going to need a proper lathe and associated tooling, or get into EDM/Electrical Discharge Machining. Either of those processes is going to require a significant time and learning investment to be proficient enough to make a gun barrel.