6x8mm Smoothbore Barrel for semi-auto builds. Is possible?

Hello, I am live in heavy restrictive country so i have no access to firearm part. I have made couple single-shot builds with simple 8mm OD 6mm ID stainless steel tube, however im unsure about using for semi-auto builds. I currently decide on how to make rifling (ECM, machining a button, carbide cutter etc) but while i acquire tools and parts, i wonder if is possible to use 6mm x 8mm stainless steel tube for semi-auto build? tube ID (6mm) wider than bullet OD (5.5mm) so is it possible to build up pressure for blow-back? perhaps is smoothbore steel tube with 5.4mm ID, instead of 6mm, any better for semi auto build? or i must use some rifled barrel in order to make semi-auto function?

secondary question: can use a printed suppressor with a 6x8mm smoothbore build? i am scared for bullet hitting baffles and blowing up suppressor, so i have not tried yet.

any help is great!! thanks friends <3